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HIGH-TENSION 

UNDERGROUND    ELECTRIC 

CABLES 

• 

A  PRACTICAL  TREATISE   FOR   ENGINEERS 


HENRY  FLOY,  M.  A.,  M.  E. 

Co?isulting  Engitieer 

MEMBER    OF 

American  Institute  of  Electrical  Engineers 
Illuminating  Engineering  Society 

New  York  Electrical  Society 
National  Jury  of  Awards,  Louisiana  Purchase  Exposition,  1904 

AUTHOR    OF 

'THE  COLORADO  SPRINGS  LIGHTING   CONTROVERSY 

Etc. 


FIRST  EDITION,    1909 

Price,  $2.00,  prepaid. 


NEW    YORK : 

ELECTRICAL    PUBLISHING    COMPANY 
165    BROADWAY 


Copyright,  1909,  by 
HENRY  FLOY 


PREFACE 


As  one  of  the  experts  in  a  recent  and  important  con- 
troversy regarding"  the  necessity  of  putting  under- 
ground certain  high-tension  aerial  wires  in  the  largest 
city  in  America,  the  author  was  made  to  realize  the 
general  lack  of  information  with  reference  to  the  pos- 
sibilities and  advantages  of  subsurface  electric  trans- 
mission. This  led  to  the  writing  of  a  series  of  papers 
on  high-tension  cables,  which  were  published  in  the 
Electrical  World,  during  the  fall  of  1908.  The  interest 
displayed  at  this  time,  in  these  matters,  has  led  the 
author  to  conclude  that  re-writing,  expanding  and  add- 
ing to  these  papers  so  as  to  compile  a  summary  of  the 
high-tension  cable  situation  as  it  exists  to-day,  would 
be  a  valuable  and  a  helpful  contribution  to  the  up-to- 
date  literature  of  electrical  engineering. 

In  the  following  pages  are  contained  summaries  of 
experience,  facts  and  figures,  which  have  been  gathered 
from  almost  innumerable  sources,  so  that  the  whole 
may  be  said  to  fairly  represent  the  concensus  of  present 
opinion  of  a  majority  of  engineers  acquainted  with  and 
practiced  in  the  use  of  high-tension  subsurface  trans- 
mission. 

After  a  brief  explanation  of  the  development  of  un- 
derground transmission,  the  verbatim  opinions  of  ex- 
perts using  such  method  of  operation  are  given  ;  also 

501.801 


6  PREFACE. 

records  made  by  various  companies  with  a  list  of  the 
more  interesting  high-tension  cable  installations,  in- 
cluding potentials  employed,  thickness  of  insulations, 
sheaths  and  other  data.  The  advantages  of  under- 
ground compared  with  aerial  construction  are  brought 
out,  followed  by  a  discussion  of  the  dielectrics  em- 
ployed and  the  present  practical  voltage  limits  attain- 
able with  electric  underground  transmission.  Curves, 
tables  and  data  are  presented  relating  to  the  heating 
and  testing  of  cables,  as  well  as  formulae  to  be  used  in 
electrical  calculations.  The  book  concludes  with  a 
chapter  on  the  costs  of  underground  installations  with 
particular  reference  to  the  prices  of  cables. 

The  author  desires  to  extend  his  thanks  to  those  who 
have  co-operated  in  his  efforts  to  compile  up-to-date 
knowledge  and  practice,  and  trusts  that  this  little 
volume  will  prove  of  assistance  to  those  who  desire  to 
acquaint  themselves  either  with  what  is  being  done  or 
what  are  the  present  possibilities  of  high-tension  un- 
derground electric  transmission. 

HENRY  FLOY. 

City  Investing  Building, 

New  York,  February  1,  1909. 


TABLE  OF  CONTENTS, 


Page 
Frontispiece,  St.  Paul  Cables. 

Preface    5 

Table  of  Contents 7 

CHAPTER  I,  INTRODUCTORY 

General    11 

Historical    14 

Expert    Opinions 15 

CHAPTER  II,  CABLE  RECORDS 

Operating    Data 18 

Other   Installations    23 

Exhibits 26 

Cables  in  Use 26 

Table  I,  List  of  Installations 28 

CHAPTER     III,     ADVANTAGES     OF     UNDER- 
GROUND CABLES 

Existing    Conditions 33 

First,   Lightning 34 

Second,  Breakdowns 34 

Third,    Interference..  36 


8  TABLE  OP  CONTENTS. 

Page 

Fourth,   Accidents    38 

Fifth,   Interruptions    38 

Comparative    Example    39 

CHAPTER  IV,  CABLE  INSULATION. 

Dielectrics  Employed  41 

Rubber  Insulation  43 

Paper  Insulation 51 

Cambric  Insulation 54 

Dielectric  Stresses 57 

Graded  Insulation 59 

Composite  Construction 61 

Thickness  of  Commercial  Insulations 63 

Table  II,  Thickness  of  Cambric  Insulation..  65 
Table  III,  Thickness  of  Rubber  and  Paper 

Insulation  66 

Table  IV.  Thickness  of  Paper  Insulation.  ...  67 
Table  V,  Thickness  of  Rubber  and  Paper 

Insulation  67 

Joints  68 

Specifications  71 

Practical  Commercial  Potentials  73 

CHAPTER  A;,  METAL  IN  CABLES. 

Copper  77 

Table  VI.  Commercial  Bare  Copper  Solid 

AYires  78 

Table  VII.  Commercial  Bare  Copper 

Stranded   Wires    ,  79 


TABLE  OF  CONTENTS.  9 

Page 
Table    \  III,    Approximate    Outside    Diameters 

of  Three-Conductor  Copper  Cables 80 

Aluminum     81 

Table   IX,  Comparative    Diameters  *  of    Bare 
Copper    and     Aluminum     Stranded     Wires 

having'  the  same  Conductivity   82 

Tin  and  Lead .  .                     83 


CHAPTER  VI,  HEATING  OF  CABLES. 

Cables,  versus  Wires   88 

A.  C.  vs.  D.  C 90 

Carrying'  Capacity    92 

Fig.  1,  Curves  of  heating  of  single-conductor 

cables 94 

Fig.  2.  Curves  of  heating  of  concentric  cables     95 
Fig.  3.  Curves  of  heating  of  three-conductor 

cable     \ 96 

Table     X.   Recommended     Current     Carrying 
Capacities  for  Cables  and  Watts  Lost  per 

Foot    97 

Table  XI.  Equivalent  Conductor  Areas 98 

Table    XII.    Recommended    Power    Carrying- 
Capacity   in    Kilowatts    99 

Table    XIII.    Current    Carrying    Capacity    of 

Three-Conductor  Cables    102 

Temporary  Loads    103 

Ducts  104 


io  TABLE  OF  CONTENTS. 

Page 

CHAPTER  VII,  ELECTRICAL  FORMULAE 
FOR  CABLES. 

Resistance    106 

Inductance     107 

Capacity    108 

Table      XIV,      Relative      Specific      Inductive 

Capacities     Ill 

Table    XV.    Insulation   Resistance    and   Elec- 
trostatic Capacity  Temperature  Coefficients  112 
Table    XVI.     Capacity    of    Three-Conductor 

Cables 115 

Table  XVII.  Cable  Capacity  Measurements.    116 

Reactance     118 

Impedance     119 

Table  XVIII.  Approximate  Ohmic  Resistance 

and    Impedance    120 

Skin   Effect    120 

CHAPTER  VIII.    TESTING  OF  CABLES. 

Summary    122 

Ohmic  and  Puncture  Tests   122 

CHAPTER  IX.    COSTS. 

Total   Costs    129 

Cable    Costs    130 

Fisr.  4,  Curves  of  Cable  Costs  .  .   135 


CHAPTER  I. 
INTRODUCTORY 


General.  The  use  of  aerial  lines  for  transmis- 
sion and  distribution  systems  was  logically  to  be  ex- 
pected in  the  early  stages  of  electrical  development  on 
account  of  the  simplicity  and  low  cost  of  construction. 
With  the  development  of  the  industry  and  the  necessity 
of  putting  wires  underground,  continuously  insulated 
conductors  were  undertaken,  which,  like  many  other 
innovations,  proved  in  some  cases  unreliable  and  un- 
satisfactory at  the  beginning;  but  with  the  develop- 
ment of  improved  processes  and  greater  perfectedness 
in  manufacture,  subsurface  cables,  even  for  high  volt- 
ages, have  come  to  be  regarded  as  reliable  as  almost 
any  other  appliance  employed  in  the  electrical  art. 

In  spite  of  the  many  instances  of  successful  installa- 
tion and  operation  of  high-tension  cables,  both  under 
ground  and  under  water,  there  exists  a  general-  lack  of 
information,  and  to  some  extent,  a  general  prejudice, 
which  prevents  their  wider  use  and  installation. 

The  importance  to  the  engineer  of  knowing  the  high- 
est practical  voltage  at  which  subsurface  cables  can  be 
successfully  operated,  the  minimum  insulation  safely 
allowable  for  a  given  potential,  and  the  cost  of  such 
cables  completely  installed,  is  not  fully  realized. 


12  INTRODUCTORY. 

Knowledge,  or  lack  of  knowledge,  of  this  subject  on 
the  part  of  the  engineer  in  charge,  may  determine 
whether  an  alternating  or  a  direct-current  system  of 
high  or  low  voltage  shall  be  selected  for  a  given  instal- 
lation with  consequent  large  or  small  expenditure  in 
transformer  plant,  elaborate  switchboard,  enlarged 
buildings,  or  unnecessarily  heavy  insulation.  The  lack 
of  general  information  regarding  recent  improvements 
made  both  in  the  manufacture  of  cables  and  in  the  solu- 
tion of  the  peculiar  troubles  like!}'  to  arise  in  the  opera- 
tion of  high-tension  cables,  probably  accounts  in  a  large 
measure  for  their  comparatively  limited  use.  There 
has  existed  no  particularly  urgent  incentive  for  investi- 
gation as  to  the  possibilities  and  advantages  to  be 
gained  in  the  construction  and  use  of  high-tension 
cables.  The  operating  companies,  in  order  to  avoid  in- 
vestment, have  usually  been  opposed  to,  and  in  conse- 
quence have  developed  and  used  every  argument 
against,  underground  construction.  The  cable  manu- 
facturers themselves,  conceding  the  good  work  done 
by  some  of  them,  have  been  working  along  commercial 
rather  than  scientific  lines,  and  the  properties  of  the 
several  dielectrics  available  for  insulation  have  been 
considered  from  a  commercial  rather  than  an  engineer- 
ing standpoint.  The  manufacturers  have  hardly  paid 
sufficient  attention  to  the  electric  phenomena  of  dielec- 
trics, a  scientific  study  of  which  would  doubtless  have 
proved  both  interesting  and  remunerative.  By  way  of 
illustration  it  may  be  said  that  few  insulated  wire  man- 
ufacturers appreciate  the  difference,  in  their  effect  on 


INTRODUCTORY.  13 

insulation,  of  alternating-  waves  of  various  forms,  or  of 
the  various  processes  used  in  the  production  of  resin 
oil  that  result  in  oils,  varying  widely  in  value,  for  use 
as  dielectrics.  Recent  research  along  these  lines  gives 
promise  of  far-reaching  results  that  mark  a  decided  ad- 
vance in  the  use  and  the  permanency  of  underground 
cables.  It  is  only  just  recently  for  example,  that  one 
has  been  able  to  purchase  paper  cables,  the  flexibility 
of  which  remains  practically  unchanged  at  zero  tem- 
perature. 

It  is  now  recognized  that  many  dielectrics  when 
freshly  produced,  make  an  excellent  showing,  but  in  the 
course  of  a  few  months  or  years,  undergo  physical  or 
mechanical  change  which  greatly  depreciates  their 
value  or  renders  them  worthless.  Permanency  has 
been  generally  admitted  as  the  sine  qua  non  of  cable 
success,  and  this  is  now  being  obtained  as  a  result  of 
experiment  and  test.  The  only  really  valid  objection 
that  can  to-day  be  urged  against  the  use  of  under- 
ground cables  is  their  relatively  high  cost  as  compared 
with  aerial  lines,  but  this  objection  decreases  in  almost 
direct  proportion  to  the  increase  in  the  number  of  cir- 
cuits installed.  Attempts  to  meet  this  objection  of 
initial  expense  have  been  made  in  several  ways,  pri- 
marily and  most  successfully  by  substituting  a  cheaper 
material,  such  as  paper  or  cambric,  for  rubber  insula- 
tion ;  and  secondarily,  by  the  construction  of  cheaper 
forms  of  conduits  in  which  the  cables  are  drawn  or  by 
the  abandonment  of  conduits  altogether,  simply  lay- 
ing the  cables  in  the  ground — as  is  being  done  at  pres- 


14  INTRODUCTORY. 

ent ;  for  example,  by  the  New  York  Edison  Company 
in  city  parks — or  in  some  cases,  where  low  potentials 
are  used,  embedding  the  bare  conductor,  in  situ,  in  a 
cheap  insulating  material,  usually  a  bitumen  com- 
pound. 

The  practicability  and  reliability  of  cables  for  110, 
220,  or  even  500-volt  service,  is  usually  admitted ;  but 
when  cables  for  higher  potentials  are  considered  it  is 
often  asserted  that  they  are  unreliable.  Contradiction 
of  such  statements  is  best  made  by  an  examination  of 
the  records  made  by  high-voltage  underground  sys- 
tems, and  the  conclusion  with  regard  thereto  submitted 
by  those  having  practical  experience  with  such  systems 
rather  than  by  the  consideration  of  statements  of  mere 
theorists  or  those  not  practically  engaged  in  the  trans- 
mission of  electrical  energy  at  high  voltages  under- 
ground, or  those  endeavoring  to  operate  such  systems 
who  have  not  the  education  or  experience  qualifying 
them  to  do  so  successfully. 

Historical.  It  is  not  perhaps  generally  appreciated 
that  26  years  ago,  underground  cables  laid  in  a  trench 
filled  with  "Bitite,"  a  vulcanized  bitumen,  were  giving 
satisfactory  service  for  low-voltage  distribution  in  Eng- 
land, and  that  25  years  ago,  Eastbourne,  England,  was 
lighted  from  the  comparatively  high-voltage  circuits  of 
the  Brush  Company  which  were  contained  in  an  under- 
ground system  of  iron  pipes  through  which  the  con- 
ductors were  drawn.  Twenty  years  ago,  2000-volt  un- 
derground cables  were  in  use  in  Rome,  Tivoli,  Turin 
and  Milan,  while  Berlin  early  had  a  reputation  for  its 


INTRODUCTORY.  15 

underground  system,  and  Paris  began  its  subsurface 
distribution  by  installing  copper  bars  supported  on 
porcelain  knobs  in  its  sewers.  The  well-known  10,000- 
volt  concentric  cables  of  Ferranti  were  installed  in  Lon- 
don over  18  years  ago  and  early  proved  the  success  of 
high-tension  underground  transmission.  Cables  with 
rubber  insulation  4/32  inch  thick  covered  with  a  lead 
sheath,  operating  on  7,500-volt  arc  light  circuits,  in- 
stalled in  Buffalo  17  years  ago,  are  still  in  use.  In 
1889,  New  York  City  had  many  miles  of  low-tension 
underground  cables  and  the  city  authorities,  resorting 
to  police  methods,  were  cutting  down  aerial  lines  to 
force  the  companies  underground.  Since  those  days, 
marked  advance  has  been  made  both  in  details  of  con- 
duit construction  and  methods  of  cable  manufacture. 

Expert  Opinions.  The  present  status  of  high-ten- 
sion underground  distribution  can  be  best  learned  from 
a  consideration  of  the  expressed  opinions  of  some  of 
the  well-known  members  of  the  engineering  profession 
who  have  attained  and  still  hold  their  high  positions, 
largely  by  reason  of  their  successful  operation  of  such 
high-tension  systems.  Mr.  L.  A.  Ferguson,  president 
of  the  American  Institute  of  Electrical  Engineers  and 
vice-president  of  the  Commonwealth  Edison  Company, 
Chicago,  which  company  has  a  station  generating 
capacity  of  120,000  kw  and  operates  both  aerial  and 
conduit  systems  including  nearly  400  miles  of  9,000  and 
20,000-volt  paper-insulated  underground  cable,  in  ad- 
dition to  much  low-tension  cable,  succintly  states  the 


1 6  INTRODUCTORY. 

superiority  of  subsurface  conductors  over  aerial  con- 
struction as  follows  :* 

"It  is  generally  conceded  that  when  the  busi- 
ness wrill  warrant  the  investment,  electrical  lines 
are  much  better  underground  than  overhead.'' 

An  ex-president  of  the  same  Institute,  l\Ir.  H.  G. 
Stott,  chief  engineer  of  the  Interborough  Rapid  Tran- 
sit Company — having  95,600-kw  station  capacity  dis- 
tributed wholly  through  375  miles  of  11,000-volt  cables, 
some  submarine — says  :f 

"1  think  it  dwells  in  the  minds  of  many  able 
engineers  that  high-tension  lines  are  very  dan- 
gerous. I  differ  from  that.  1  think  the  high- 
tension  underground  cable  is  the  safest  thing  we 
have — a  great  deal  safer  than  low-tension." 

Mr.  J.  W.  Lieb,  Jr.,  also  ex-president  of  the  American 
Institute  of  Electrical  Engineers  and  general  manager 
of  the  Xew  York  Edison  Company — having  150,000  kw 
rated  generating  station  capacity,  or  including  storage 
batteries,  200,000  kw  capacity — which  company,  in  ad- 
dition to  many  miles  of  low-tension  cable,  is  operating 
over  200  miles  of  6,600-volt  cables,  stated  to  the  writer 
with  reference  to  the  high-tension  cables,  that, 

"There  is  no  question  as  to  the  practicability 
and  reliability  of  underground  cables  whether 
for  lowr-tension  or  high-tension  service,  when 
compared  with  aerial  conductors." 

*  Paper  presented  at  the  International  Congress,  St.   Louis,   1904, 
entitled,  "  Underground  Electrical  Construction." 
f  Proceedings  A.  I.  E.  E.,  vol.,  XXI.,  page  443. 


INTRODUCTORY.  17 

Warren  Partridge,  Engineer  for  the  Public  Service 
Corporation  of  New  Jersey,  says* 

"In  spite  of  all  difficulties  experienced****** 
cable  systems  are  fully  as  reliable  .as  other  ele- 
ments in  the  electric  power  system.  Our 
records  for  a  period  of  three  years  show  that 
cable  breakdowns  caused  but  7  per  cent  of  all  in- 
terruptions to  service  and  that  the  duration  of 
time  of  cable  interruptions  was  no  longer  than 
the  average  interruptions  from  other  causes." 

Many  other  less  well-known  but  equally  enthusiastic 
believers  in  the  use  of  subsurface  conductors,  could  be 
cited,  if  further  argument  were  necessary. 


*  Proceedings  A.  I.  E.  E.,  vol.  XXVIII.,  page  106. 


CHAPTER  II. 

CABLE    RECORDS 


Operating  Data.  Reference  to  the  records  of  break- 
clowns  on  high-tension  cables  in  actual  use,  substan- 
tiates the  claim  to  reliability  for  high-tension  cables. 

Mr.  Peter  Junkersfeld,  referring  to  the  Chicago  sys- 
tem, says  their  cable  troublesf 

"during  the  last  three  years  have  averaged 
only  two  cases  per  hundred  miles  per  year. 
This  includes  all  troubles  on  9,000-volt  cables 
from  known  or  unknown  causes,  except  those 
due  to  external  injury  to  the  lead  sheaths." 

In  a  more  recent  paperj  Mr.  Junkersfeld  shows  that 
during  the  preceding  five  and  a  half  years,  their  9,000 
and  20,000-volt  cables,  aggregating  275  miles,  had  a 
total  of  only  48  breakdowns,  of  which  26  were  due  to 
external  causes ;  or,  ignoring  damage  from  external 
causes,  there  was  only  one  break-down  per  year  per  15 
miles  of  cable  installed.  Of  the  total  number  of  burn- 
outs, but  a  small  percentage  caused  any  serious  shut- 
downs, and  the  company  is  now  engaged  in  extending 
its  underground  system  by  adding  68  miles  of  9,000- 
volt,  250,000-cm,  three-conductor  cable,  and  44  miles  of 


f  Proceedings  A.  I.  E.  E.,  vol.  XX VI..  page  1614.      Part  II. 
\  Proceedings  A.  I.  E.  E.,  vol.  XX VI I.,  page  i^/o. 


CAB LH  RECORDS.  19 

additional  20,000-volt,  No.  00,  B.  &  S.  three-conductor 
cable.  The  9,000-volt  cables  are  insulated  with  6/32 
inch  paper  about  each  conductor  and  a  jacket  or  belt 
of  4/32  inch  paper;  the  20,000-volt  cables  with  9/32 
inch  paper  around  each  conductor  and  a  belt  of  6/32 
inch  over  all. 

The  Interborough  Rapid  Transit  Company,  after 
several  years  of  operation,  found  it  averaged  only  one 
breakdown  per  year  per  62^  miles  of  cable,  including 
the  larger  number  of  interruptions  liable  to  incur  on 
new  installations.*  Their  Chief  Engineer  recently 
said  :f 

"The  number  of  burnouts  per  100  miles  of 
cable  per  annum,  has  fallen  during  the  last  two 
years  to  0.28,  or  practically  one  fault  per  400 
miles  of  cable  per  year.  That  is  a  reassuring 
record ;  when  our  overhead  transmission  lines 
can  show  anything  like  it,  we  can  look  forward 
to  reliable  long-distance  transmission." 

The  New  York  Edison  Company  has  never  had  a 
complete  shutdown  of  its  system  from  any  cause  during 
the  past  15  years,  which,  of  course,  includes  its  200 
miles  of  6,600-volt  system.  Despite  the  difficulties  en- 
countered in  making  installations  in  the  streets  of  New 
York  and  the  early  period  at  which  much  of  its  under- 
ground system  has  been  installed,  this  company  has 
had  only  66  cable  breakdowns  of  all  kinds  during  the 


*  Proceedings  A.  T.  E.  E.,  vol.  XXVI..  page  1641.     Part  II. 
f  Proceedings  A.  1.  E.  E.,  vol.  XXVI 1 1. ,  page  96. 


20  CABLE  RECORDS. 

nine  years  of  high-tension  operation.  Of  these  break- 
downs, 32  developed  during  operation  and  34  were 
found  either  by  periodic  insulation  tests  or  by  inspec- 
tions of  the  cables.  Of  the  32  that  developed  during 
operation,  only  18  were  caused  by  other  than  mechani- 
cal injuries,  which,  based  on  200  miles,  makes  a  record 
of  one  breakdown  per  year  per  100  miles  of  cable  oper- 
ated.* 

Mr.  Charles  E.  Phelps,  chief  engineer  of  the  Electri- 
cal Commission  of  the  City  of  Baltimore,  Md.,  shows 
that  the  breakdowns  of  all  the  various  cables — includ- 
ing telephone,  fire  and  police  service — amounting  in 
1906  to  nearly  300  miles,  operating  in  Baltimore  under 
various  potentials  and  as  high  as  13,000  volts,  were  148 
for  a  period  covering  seven  years ;  or,  omitting  the 
years  1903-4-5,  when  the  breakdowns  were  abnormally 
high  owing  to  street  improvements  consequent  upon 
the  fire  and  electrolytic  action,  there  is  an  average  of 
about  one  breakdown  per  year  per  40  miles  of  cable  of 
all  kinds. 

In  Buffalo,  where  for  years  they  operated  11,000-volt 
cables  with  commercial  satisfaction  and  published  rec- 
ords on  two  of  their  9/32  rubber-insulated,  three-con- 
ductor No.  000,  B.  &  S.  with  no  over-all  jacket,  lead- 
covered  cables,  each  about  6  miles  long  show  only  two 
break-downs,  these  from  mechanical  injury,  from  1900 
to  19064  The  Public  Service  Corporation  of  New  Jer- 
sey, has  about  90  miles  of  underground  and  65  miles 


*  Proceedings  A.  I.  E.  E.,  vol.  XX VI.,  page  1615,  Part  II. 
J  Proceedings  A.  I.  E.  E.,  vol    XXV.,  page  209. 


CABLE  RECORDS.  2 1 

of  overhead  cables  operating  at  13,200  volts,  most  of 
them  being-  No.  00,  B.  &  S.  All  these  cables  are 
three-conductor,  paper-insulated  7/32  inch  over  each 
conductor,  7/32  inch  over  all  with  1/8  inch  lead  sheath. 
The  breakdowns  from  January  1,  1905t  to  October  1, 
1908,  3.75  years,  were,  11  in  joints,  16  from  external 
causes,  25  in  cables,  a  total  of  52.  Thus  the  break- 
downs, excluding  external  causes  and  defective  in- 
stallation, are  10  miles  of  cable  per  breakdown  per 
year.  Half  the  total  number  of  cables  had  no  trouble 
whatever ;  5  cables  had  1  each ;  2  cables  had  2  each ; 
and  4  cables  had  16  breakdowns,  the  latter  being  tie- 
lines  not  straight  feeder-lines. 

In  a  paper  read  before  the  Pittsburg  Branch  of  the 
American  Institute  of  Electrical  Engineers,  May  8, 
1907,  Mr.  Charles  W.  Davis,  reported  figures  relating 
to  operating  breakdowns  on  1,462,000  feet  of  three-con- 
ductor lead-covered  underground  cable  with  potentials 
of  from  11,000  to  16,000  volts  installed  on  14  different 
"construction  jobs."  The  number  of  breakdowns  of 
all  kinds  were  15,  or  one  breakdown  in  joint  for  every 
324  made ;  one  breakdown  in  bends  in  manholes  for 
every  340,000  feet  of  cable,  and  one  breakdown  for 
every  227,000  feet  of  cable  lying  wholly  within  ducts. 
Taking  into  consideration  the  four  years  covered  by 
the  breakdowns,  there  were  from  all  causes  whatsoever 
one  breakdown  per  year  per  390,000  feet  (74  miles)  of 
cable.  Mr.  Davis  concludes,  his  paper  with  the  state- 
ment that  the  figures  indicate  that 


22  CABLE  RECORDS. 

"practically  all  the  defects  or  faults  existing  in 
a  system  will  be  weeded  out  by  the  initial  high- 
voltage  tests,  the  remaining  few,  if  such  still  ex- 
ist, being  developed  by  the  first  few  months  of 
regular  service.  This  conclusion  is  confirmed 
by  observations  on  many  other  installations  not 
covered  by  these  remarks." 

Among  the  examples  of  less  extensive  installations 
than  those  referred  to  above,  may  be  mentioned  the 
Twin  City  Rapid  Transit  Company,  of  Minneapolis, 
Minn.,  which  has,  at  present,  some  60  miles  of 
three-conductor  13,000-volt,  paper-insulated  cable, 
much  of  which  has  been  operating  since  1897,  and 
during'  the  last  three  years  it  has  had  a  total 
of  only  six  breakdowns  due  to  other  than  mechan- 
ical injury  or  poor  workmanship.  Two  three- 
conductor  cables,  one  with  paper  insulation  and 
the  other  with  rubber  insulation,  were  installed  in 
St.  Paul,  Minn.,  in  1890,  for  25,000-volt  service  and  have 
been  giving  satisfactory  results  under  rather  exacting 
conditions,  Although  the  first  underground  installa- 
tion made  for  operating  potentials  anything  like  as  high 
as  25,000  volts,  at  a  time  when  there  was  considerably 
less  knowledge  and  experience  with  high-tension  work, 
these  St.  Paul  cables  have  a  total  record  of  but  37  fail- 
ures from  all  causes  in  nearly  eight  years  of  con- 
tinuous service,  and  33  per  cent  of  all  the  failures 
occurred  in  one  year,  due  mainly  to  special  difficulties. 
The  cables  are  connected  to  the  end  of  a  24-mile  aerial 


CABLE  RECORDS.  23 

transmission  line  and  are  possibly  therefore  particu- 
larly subject  to  lightning. 

At  Montreal,  Canada,  four  three-conductor  cables  are 
operated,  each  about  4,500  feet  long,  at  25,000  volts. 
These  cables  were  installed  in  1902,  and  during  the  six 
years  intervening  to  date  only  eight  breakdowns  in  all 
have  been  reported,  although  for  part  of  their  length 
they  are  installed  in  ducts  under  a  canal. 

There  is  a  second  installation  under  the  St.  Lawrence 
River  at  Montreal,  of  three-conductor  and  single-con- 
ductor rubber-insulated  cables  operating  at  25,000 
volts,  part  of  which  was  made  in  1906,  and  although 
connected  to  aerial  lines  and  operating  under  water, 
only  a  total  of  one  breakdown  from  all  causes  has  been 
reported  to  date.  The  same  company  is  also  operating 
satisfactorily  several  12,500-volt  submarine  cables. 

Other  Installations.  At  York  Haven,  Pa.,  in  1906, 
were  installed  two  three-conductor  rubber-insulated 
armored  cables,  each  3,280  feet  in  length,  which  have 
been  operating  continuously  at  25,000  volts.  These  ca- 
bles, between  the  generating  station  and  one  end  of  an 
aerial  transmission  line,  are  laid  under  water  across  the 
Susquehanna  River.  Philadelphia  has  about  100  miles 
of  three-conductor  lead-covered  cables  and  Baltimore 
over  125  miles  of  No.  000  B.  &  S.  three-conductor 
paper-insulated  cables,  all  being  operated  at  13,200 
volts  under  ground.  In  New  Orleans,  where  the  ducts 
are  more  or  less  continuously  filled  with  water,  there 
are  about  8  miles,  and  in  Boston  and  Washington,  D. 
C.,  many  miles  of  6,600-volt  cable.  In  Portland,  Ore., 


24  CABLE  RECORDS. 

an  ll,COO-volt  submarine  cable  is  in  use.  San  Fran- 
cisco, Cal.,  has  been  using  11,000-volt,  three-conductor 
cable  with  "graded"  insulation,  about  10  years.  In 
New  York  City,  the  passenger  service  of  all  railroads 
is  operated  entirely  therein  by  electricity  supplied  at 
11,000  volts  through  three-conductor  lead-covered 
cables  that  are  partly  submarine  and  partly  under- 
ground, or  in  iron  pipes;  in  the  Borough  of  Queens,  an- 
other railroad  system  depends  for  the  operation  of  a 
large  part  of  its  service  upon  11,000-volt  underground 
cables. 

Under  the  Hudson  River  at  Poughkeepsie,  N.  Y., 
there  are  two  three-conductor  rubber  cables  operating 
at  12,000  volts,  and  at  Houghton,  Mich.,  similar  sub- 
marine cables  are  operated  at  the  same  voltage. 
Across  Great  Bay,  at  Portsmouth,  X.  II. ,  there  are  two 
three-conductor  rubber  submarine  cables,  5,000  feet  in 
length,  operating  at  13,000  volts ;  and  at  Norfolk,  Va., 
4,000  feet  of  three-conductor  submarine  cable  operating 
at  11,000  volts.  Berlin  is  using  three-conductor,  steel 
taped  10,000-volt  cables.  In  both  Toronto  and  Quebec, 
Canada,  and  Providence,  R.  I.,  12,000-volt  underground 
cables  are  in  use.  Detroit,  Michigan,  is  operating  two 
No.  2,  B.  &  S.  three-conductor  cables  each  7.5  miles 
long,  at  a  potential  of  23,000  volts,  and  insulated  with 
2/32  inch  rubber  plus  6/32  inch  varnished  cambric 
about  each  conductor  with  a  jacket  of  3/32  inch  cambric 
and  a  3/32  inch  lead  sheath.  In  Rio  Janeiro,  Brazil,  13,- 
000-volt  cables  have  recently  been  installed,  and  about  a 
year  ago,  there  were  put  in  operation  in  Durham  and 


CABLE  RECORDS.  25 

Northumberland  Counties,  England,  nearly  100  miles 
of  20,000-volt,  three-conductor  cable,  with  a  consider- 
able additional  mileage  of  12,000  and  6,000-volt  cables, 
all  of  which,  at  last  reports,  \vere  operating  satisfac- 
torily. 9 

Spain  has  installed  some  cables  operating  at  15,000 
volts,  while  in  Italy  they  are  using  10,000-volt  cables 
at  Naples,  12,000-volt  cables  at  Genoa  and  16,000-volt 
cables  at  Milan,  and  at  the  end  of  aerial  lines  some  20,- 
000,  25,000  and  even  30,000-volt  underground  cables. 

The  Moutiers-Lyons,  France,  continuous  current, 
60,000-volt  transmission  line  feeds  into  two  substa- 
tions at  Lyons,  which  are  about  2T/2  miles  apart,  and 
connected  in  series  through  single-conductor  under- 
ground cables.  The  cables  have  a  section  of  75  sq. 
m.  m.,  and  after  being  insulated,  are  protected  with 
both  a  lead  covering  and  steel  armoring. 

The  above  mentioned  installations,  although  only 
a  partial  list,  indicate,  to  some  extent,  the  present-day 
wide  use  and  exacting  requirements  made  of  high-ten- 
sion cables.  The  successful  employment  of  high-ten- 
sion cables  under  water  is  particularly  interesting,  be- 
cause of  a  popular  belief  that  the  use  of  high-tension 
cables  under  such  conditions  is  almost  impossible. 
Furthermore,  such  cables  are  often  installed  at  the  end 
or  in  the  middle  of  a  high-tension  line,  so  that  they  are 
particularly  subject  to  damage  by  lightning  or  the  pil- 
ing up  of  potential  due  to  a  change  in  the  constants  of 
the  transmission  line.  Although  the  installations  cited 
above  refer  only  to  constant  potential  circuits,  it  is 


26  CABLE  RECORDS. 

well  known  that  there  are  miles  of  underground  series 
arc  circuits  in  most  large  American  cities  nightly  carry- 
ing potentials  of  from  6,000  to  10,000  volts. 

Exhibits.  At  the  Louisiana  Purchase  Exposition  at 
St.  Louis,  1904,  samples  of  cables  designed  for  50,000 
volts  (effective)  and  tested  to  100,000  volts  without  per- 
foration, were  exhibited.  Similar  cables  were  shown  at 
the  Milan  Exposition  in  1906,  which,  being  tested  for 
breakdown  point,  in  about  15-feet  lengths,  gave  way  at 
slightly  above  200,000  volts. 

The  cable  manufacturers  in  America  and  abroad  are 
prepared  to  furnish  what  may  fairly  be  termed  high- 
tension  cables.  Some  makers  are  prepared  to  supply 
and  guarantee  cable  for  40,000  to  50,000-volt  service, 
while  one  reliable  manufacturer  has  submitted  the 
writer  a  bona  fide  proposition  for  a  client,  to  furnish 
single-conductor  cables,  lead-sheathed,  for  75,000-volt 
service,  pieces  cut  off  to  withstand  a  test  of  150,000 
volts,  the  price  being  comparable  with  that  of  a  cable 
designed  for  more  moderate  voltages.  One  American 
manufacturer  reports  the  production  of  a  small  amount 
of  cable  for  commercial  service,  which  satisfactorily 
withstood  time  tests  of  150,000  volts  and  required 
about  240,000  volts  to  break  down. 

Cables  in  Use.  Below  is  given  a  table  showing  some 
of  the  most  interesting  high-voltage  underground  and 
submarine  cable  installations  in  America,  together  with 
information  as  to  the  method  of  operating  the  character 


CABLE  RECORDS.  27 

and  thickness  of  insulation,  insulation  per  thousand 
volts,  etc.  It  will  be  noted  that  the  total  thickness  of 
insulation  per  thousand  volts  between  conductors 
varies  from  over  4/6-i  inch  in  6,600-volt  cables,  down  to 
about  1/64  inch  in  25,000-volt  cable.  This  differ- 
ence is  due  to  four  different  causes : 

First.  Difference  in  their  value  as  dielectrics,  of  the 
materials  employed  as  insulation. 

Second.  Ignorance  as  to  the  minimum  insulation 
that  is  safe  for  a  given  potential. 

Third.  Difference  of  opinion  among  engineers  as  to 
the  proper  factor  of  safety  to  use  in  the  design  of  high 
potential  cables. 

Fourth.  The  higher  the  operating  voltage  probably 
the  less  the  proportionate  increase  in  temporary  poten- 
tial strains  due  to  surges,  etc. ;  and  hence,  the  less  the 
necessity  for  a  high  factor  of  safety. 

The  list  of  35  distinct  installations  hereafter  given, 
aggregating  over  2,300  miles  of  high-tension  cable — 
operated  under  so  many  diverse  circumstances  as  to 
voltage,  current  carrying  capacity,  character  of  insula- 
tion, outside  metal  protection,  with  widely  varying 
electrical  and  climatic  conditions — demonstrates  that 
subsurface  electrical  transmission  at  high  voltages  can- 
not be  considered  in  any  sense  experimental. 


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CHAPTER  III. 

ADVANTAGES    OF  UNDERGROUND 
CABLES. 


Existing  Conditions.  In  American  cities  and  towns 
of  any  considerable  size  the  local  regulations  usually 
require  that  all  wires  be  put  under  ground,  except  in 
the  more  sparsely  populated  portions.  Where  such  re- 
quirements exist,  the  distribution  of  electrical  energy 
is  at  present  being  generally  done  by  means  of  lead- 
covered  cables  threaded  through  vitrified  clay,  wood 
fiber  or  bituminized  paper  ducts  laid  in  Portland  ce- 
ment. By  far  the  greater  proportion  of  work  installed 
under  the  conditions  cited  is  for  low-potential  distri- 
bution, although  in  the  larger  cities  and  as  a  section  of 
a  transmission  line  entering  such  a  city  district  or  pass- 
ing under  a  river,  many  high-tension  underground  in- 
stallations can  be  shown.  The  relative  advantages  of 
high-tension  underground,  as  against  aerial  construc- 
tions, cannot  perhaps  be  properly  considered  in  such 
connection,  because  subsurface  construction  is  more 
or  less  compulsory.  It  has,  moreover,  rather  been  the 
practice  of  engineers  not  to  resort  to  the  use  of  high- 
tension  underground  installations  except  under  some 
such  compulsory  conditions.  As  the  advantages  and 
reliability  of  high-tension  cable  construction  are 
realized,  a  wider  use  of  such  cables  is  sure  to  result. 


34  ADVANTAGES  OF  UNDERGROUND  CABLES. 

Among-  the  advantages  are  : 

First,  Lightning.  Absolute  freedom  from  interrup- 
tion of  service  and  damage  to  apparatus  from  lightning 
disturbances.  It  is  generally  recognized  and  acknowl- 
edged that  any  system  of  electrical  distribution  which 
is  completely  underground  is  immune  from  atmos- 
pheric lightning,  although,  of  course,  disturbances  and 
undue  potentials  that  arise  by  reason  of  surges,  arcing 
grounds,  etc.,  may  occur  with  underground  as  with 
aerial  systems. 

Second,  Breakdowns.  Less  liability  of  interruption 
of  service  from  breakdowns. 

"In  Xew  York  where  probably  there  is  more 
cable  than  any  other  city  in  the  United  States, 
or  in  the  world,  interruptions  of  service  due  to 
the  breaking  down  of  a  cable  are  almost  unheard 
of."* 

Most  of  the  breakdowns  occurring  in  high-tension 
cables  are  the  result  of  "human  frailty,"  which  can  be 
largely  anticipated  and  avoided. 

"As  a  rule,  more  trouble  will  develop  on  un- 
derground cables  due  to  poor  work  on  installa- 
tion rather  than  to  faults  in  the  cables  them- 
selves, "f 

Engineering  opinion  is  practically  unanimous  in  the 
statement  that  the  weakest  point  of  cable  installation 


*  C.  W.  Rice,  Proceedings  A.  I.  E.  E.,  vol.  XXIV.,  page  416. 
f  I.  A.  Ferguson    "  Underground  Electrical  Construction  ",  Proceed- 
ings International  Electrical  Congress  of  St.  Louis,  1904. 


ADVANTAGES  OF  UNDERGROUND  GABLES.  35 

is  the  joint.  It  is  essential  that  the  insulation  at  the 
joint  shall  exclude  air  and  moisture,  and  be  as  solid 
and  perfect  as  the  balance  of  the  insulation.  To  indi- 
cate the  perfection  of  workmanship  and  material  which 
may  be  attained  by  due  care,  it  is  said  with  regard  to 
the  method  of  making  cable  joints  employed  by  the 
Commonwealth  Edison  Company,  of  Chicago,  that* 

"the  method  (see  page  70)  has  been  tried  for 
six  years  on  an  installation  comprising  420  miles 
of  high-tension  cables.  (This  figure  includes 
4400-volt  circuits.  Ed.)  During  the  entire  term 
of  this  test  only  one  failure  of  a  cable  joint 
occurred  on  these  lines,  and  this  was  plainly  at- 
tributable to  external  causes." 

Surprising  as  it  may  seem  at  first  thought,  experience 
shows  that  the  short-circuiting  or  grounding  of  a  high- 
tension  cable  results  in  as  little  as  or  less  damage  than 
in  the  case  of  low  voltage  cables.  With  low  voltages 
and  large  currents,  the  burning  resulting  may  be  seri- 
ous, and  in  at  least  one  instance  brought  to  the  author's 
attention,  damaged  several  miles  of  cable ;  whereas, 
with  high  voltages,  the  arc  is  so  severe  as  to  promptly 
extinguish  itself  or  open  the  station  safety  devices 
without  burning  more  than  perhaps  two  to  five  feet 
of  cable. 

On  account  of  the  increased  cost  of  cables  with  high 
factors  of  safety,  there  is  a  strong  tendency  to  reduce 
the  thickness  of  insulation  and  thereby  the  cost,  but  at 


*  Electrical  World,  page  544,  Sept.  5,  1908. 


36  ADVANTAGES  OF  UNDERGROUND  CABLES. 

an  increased  risk  of  breakdowns.  This  is  being  coun- 
teracted by  some  manufacturing  companies  through 
the  adoption  of  the  same  business  methods  of  installing 
high-tension  underground  cables  that  were  employed 
and  still  are,  to  some  extent,  used  in  connection  with 
the  installation  of  storage  batteries,  namely,  furnishing, 
drawing  in  and  connecting  up  the  cables  complete,  then 
undertaking  to  maintain  them,  as  against  defects  in 
manufacture  or  installation,  for  an  annual  charge, 
which  in  some  cases,  is  as  low  as  one-half  of  one  per 
cent  of  the  total  cost  of  the  cables. 

Third,  Interference.  Fewer  interruptions  of  service 
from  extraneous  interference.  Short  circuits  and 
grounds  are  more  or  less  continually  occurring  with 
aerial  lines  due  to  breaking  of  mechanically  weak  wires 
or  insulators,  storms  of  wind  and  ice,  objects  falling 
across  the  wires  and  short-circuiting  them,  and 
malicious  interference.  With  underground  con- 
ductors, annoyances  of  the  above  character  are  almost 
entirely  done  away  with,  the  cables  usually  being  in- 
stalled in  ducts  of  tough  material,  enclosed  in  concrete 
several  inches  thick,  the  whole  being  from  one  and  a 
half  feet  to  three  feet  below7  the  surface  of  the  ground, 
except  at  manholes,  which  are  protected  by  double, 
heavy  iron  covers,  affording  protection  against  almost 
anything  but  dynamite.  In  case  of  strikes,  it  would  be 
much  easier  to  patrol  and  protect  lines  in  conduits  than 
those  carried  on  poles,  because  the  latter  can  be  dam- 
aged from  a  distance  by  rifle  shots  or  wires  thrown 


ADVANTAGES  OF  UNDERGROUND  GABLES.  37 

over  the  lines,  whereas  underground  construction  must 
be  directly  approached  before  it  can  be  injured. 

"In  the  overhead  system  (Boston),  the 
troubles  are  ten  to  one  in  comparison  with  the 
underground  cable  system,  almost  all  of  these 
occuring  in  cable  newly  installed."* 

Operators  of  aerial  circuits  usually  do  not  keep  as 
full  and  complete  records  of  interferences  caused  by 
failures  of  their  lines  as  do  those  in  charge  of  cable 
installations.  Lack  of  explicit  information  frequently 
leads  those  operating  overhead  lines  to  the  conclusion 
that  their  interruptions  are  not  anything  like  as  fre- 
quent as  are  troubles  in  cable  circuits,  indicated  by 
records  that  have  been  published.  The  following 
record  for  the  last  twelve  months  furnished  by  a  com- 
pany maintaining  detailed  accounts  of  each  shutdown 
of  their  aerial  line  may  be  taken  as  indicating  results  at 
least  as  favorable  as  the  average,  because  the  line  is 
located  too  far  south  to  be  ever  troubled  by  snow  or 
ice,  is  well  built  and  operated  under  independent,  pro- 
gressive management.  The  line  is  a  little  over  100 
miles  long  and  shows  one  breakdown  per  year  for  each 
6  miles  of  line. 

It  must  be  admitted  that  a  breakdown  in  a  cable  is 
more  serious  than  in  an  aerial  line,  because  the  latter 
can  be  repaired  more  quickly  and  with  cheaper  labor 
than  the  former ;  but  breakdowns  in  cable  systems  are 


*  Proceedings  A.  J.  E.  E.,  Jan.,  1909,  page  14. 


38  ADVANTAGES  OF  UNDERGROUND  CABLES. 

not  near  as  frequent  as  interruptions  of  service  with 
aerial  lines,  despite  the  fact  that  many  high-tension 
cables  are  operating  with  rather  light  insulation  for  the 
service. 

Fourth,  Accidents.  The  liability  of  accident  to  the 
public  with  consequent  damage  suits  is  almost  entirely 
removed.  The  only  likelihood  of  danger  is  from  ex- 
plosions of  gas  that  may  accumulate  in  ducts  or  man- 
holes, but  with  proper  design  and  construction  this 
danger  can  be  practically  eliminated.  Frequent  injury 
or  death  to  individuals  coming  in  contact  with  broken 
aerial  conductors  are  too  much  of  an  every-day  occur- 
rence to  need  any  argument  to  prove  the  desirability  of 
underground  construction  from  the  standpoint  of  safe- 
ty to  the  public,  regardless  of  aesthetic  considerations 
or  the  inconvenience  of  poles  in  streets. 

The  danger  to  employees  is  less  with  underground 
than  aerial  construction  for  the  reason  that  in  repairing 
or  stringing  new  aerial  lines  there  are  usually  other  live 
circuits  on  the  same  pole  with  which  the  workmen  may 
come  in  contact,  whereas  with  underground  construc- 
tion, the  live  cables  are  enclosed  in  lead  sheaths  which 
are  grounded,  and  therefore,  harmless. 

Fifth,  Interruptions.  "With  aerial  circuits,  interrup- 
tions in  the  continuity  of  the  transmission  system 
usually  occur  without  any  forewarning.  In  contra-dis- 
tinction  to  this  the  weakening  of  the  insulation  of 
cables  is  often  determined  by  tests  or  by  suitably  de- 
signed indicating  instruments,  sufficiently  in  advance  of 


ADVANTAGES  OF  UNDERGROUND  CABLES.  39 

the  actual  breaking-  clown  of  the  insulation  to  allow 
transferring  to  another  cable  without  interruption  of 
the  service.  There  has  been  developed  by  Messrs. 
Torchio  and  Varley,  of  New  York,  a  device  now  in 
commercial  use  which  takes  into  account  the  unbalanc- 
ing of  the  condenser  capacity  current,  when  the  insula- 
tion of  a  conductor  begins  to  depreciate,  and  gives 
warning  of  approaching  danger  sufficiently  in  advance 
of  a  breakdown  to  allow  the  cable  to  be  disconnected. 

Comparative  Example.  As  illustrative  of  the  rela- 
tive cost  of  aerial  and  underground  constructions,  the 
following  figures  are  given,  having  been  prepared  in 
connection  with  plans  for  an  actual  installation  of  un- 
derground cables  for  the  transmission  of  20,000  H.  P., 
15  miles  across  country,  from  a  certain  hydro-electric 
station  to  a  substation  in  a  neighboring  city.  By  the 
use  of  conduits  laid  in  the  highways,  the  cost  of  expen- 
sive rights-of-way,  real  estate,  building  and  lowering 
transformers  for  a  substation  at  the  outskirts  of  the 
city  and  liability  of  interference  with  the  circuits  will  be 
avoided.  While  the  increased  cost  of  the  underground 
construction  seems  large  compared  with  aerial  lines, 
the  difference  will  be  only  a  small  percntage  of  the  total 
cost  of  the  complete  system,  as  noted  in  the  following 
table  : 

COMPARATIVE  COSTS  OF  SYSTEMS. 
AERIAL  LINES. 

Private  right  of  way  across  country $45,000 

Steel  towers  with  three  circuits,  complete  for 

15  miles,  ,  85,000 


4o  ADVANTAGES  OF  UNDERGROUND  CABLES. 

Three  miles  of  8-cluct  conduit,  at  $7,000  per 

mile,  21,000 

Six  10,000-volt  paper  cables  (1  spare),  at  $1.10 

per  ft., 105,000 

Substation  complete,  with  18,000-kw  trans- 
former capacity  95,000 


$351,000 

UNDERGROUND  LINES. 

Eighteen  miles    of  4-clnct  conduit,    at  $5,000 

a   mile $90,000 

Three  25,000-volt  paper  cables   (1  spare),  at 

$1 .50  per  foot, 428,000 


$518,000 

The  above  example  illustrates,  of  course,  only  one 
set  of  conditions.  The  use  of  cables  designed  for  other 
voltages  than  those  specified  would  naturally  result 
in  different  total  costs.  Again,  when  considering  the 
cost  of  transmission  lines  in  connection  with  station 
apparatus,  it  might  be  found  advisable  to  generate 
at  15,000  volts  and  transmit  at  that  voltage,  thereby 
avoiding  the  cost  of  step-up  transformers,  included  in 
the  estimates  based  on  25,000  volts  for  transmission. 
The  principle  point  to  be  brought  out  by  the  figures 
is  that  the  use  of  high-tension  underground  cables  is 
not  limited  to  city  streets ;  but,  under  certain  circum- 
stances, may  be  advantageously  used  across  country. 


CHAPTER  IV* 
CABLE   INSULATION 


Dielectrics  Employed.  While  it  should  not  be 
the  duty  of  the  purchaser  to  attempt  to  specify  the 
details  of  insulation  manufacture  and  application  any 
more  than  he  attempts  to  specify  all  the  details  of  con- 
struction of  standard  apparatus  used  in  the  electrical 
business,  nevertheless,  in  the  present  state  of  the  art 
it  seems  necessary  that  the  engineer  be  well  informed 
as  to  properties  and  limits  of  cable  dielectrics  and  their 
methods  of  production  and  application  so  as  to  be  able 
to  control  the  manufacturer,  unless  the  latter  is  willing 
to  assume  all  responsibility  for  his  product,  backing 
that  up  with  a  guarantee  covering  a  long  period  of 
years. 

The  early  attempts  in  America  at  operating  con- 
ductors underground,  were  made  in  the  larger  cities  by 
the  Edison  Companies.  The  original  system  consisted 
of  iron  pipe,  usually  in  20  feet  lengths,  containing  cop- 
per rods  covered  with  a  light  cotton  or  jute  insulation 
embedded  in  bituminous  compounds.  The  pipes  were 
called  tubes,  and  contained  two  conductors  for  feeders 
and  three  conductors  for  the  three-wire  mains ;  the 
conductors  were  united  at  their  ends  by  means  of  flexi- 
ble conductors  enclosed  in  cast  iron  couplings  or  junc- 
tion boxes  filled  with  compound  similar  to  the  tubes. 


42  CABLE  INSULA TION. 

Even  for  the  low  potentials  employed  by  the  Edison 
Companies,  the  type  of  insulation  available,  with  the 
low  melting"  point  of  the  compound,  was  soon  found  in- 
sufficient and  unsatisfactory  for  subsurface  work  and 
resulted  in  the  adoption  of  rubber  and  gutta  percha  for 
underground  insulation. 

The  lack  of  flexibility  and  accessibility  in  a  system 
where  the  conductors  could  only  be  examined  or  with- 
drawn by  tearing  up  the  streets,  developed  nearly  25 
year  ago,  the  drawing-in  system,  namely,  the  use  of 
ducts  united  by  manholes  permitting  the  drawing  in 
of  a  thoroughly  insulated,  usually  lead-covered,  flexible 
conductor.  Although  the  duct  system  has  become  al- 
most universally  accepted  and  adopted  in  America,  the 
solid  system  is  still  being  used  with  satisfactory  results 
abroad,  and  for  lower  voltages  and  special  installations 
may,  in  many  cases,  be  desirable.  Recent  improve- 
ments in  the  quality  of  the  insulating  and  water-proof- 
ing compounds  with  reductions  in  their  price,  may  yet 
see  the  more  extended  use  in  America  of  the  solid 
system,  which  permits  the  installation  of  a  bare  con- 
ductor in  situ. 

As  increasingly  higher  potentials  were  attempted, his- 
tory shows  that  rubber  and  its  compounds  were  almost 
exclusively  used  for  insulation  ;  however,  on  account 
of  its  high  cost  substitutes  were  quickly  sought  and 
paper  carefully  applied  and  properly  treated  was  soon 
found  to  be  satisfactory,  provided  dampness  and  mois- 
ture could  be  kept  away.  This  was  accomplished  by 
inclosing  the  insulation  in  a  lead  sheath,  which,  as  long 


CABLE  INSULA  TION.  43 

as  it  remains  intact,  allows  paper  insulated  cables  to 
give  very  satisfactory  service  for  the  highest  voltages 
yet  commercially  employed  under  ground.  More  re- 
cently, shellaced  cambric  has  been  used,  which,  al- 
though more  costly  than  paper  is  less  .expensive  than 
rubber,  and  unlike  the  former  does  not  quickly  depreci- 
ate in  the  presence  of  water.  The  latest  development 
is  "graded"  insulation,  which  is  a  combination  of  differ- 
ent dielectrics  or  the  use  of  a  nonuniform  material. 

Rubber  Insulation.  Rubber,  the  unique  vegetable 
product,  for  which  no  full  substitute  has  ever  been 
found,  makes  an  unsurpassed  dielectric  when  properly 
treated,  by  reason  of  its  insulating  qualities,  extreme 
flexibility  and  imperviousness  to  moisture.  Crude  rub- 
ber varies  widely  in  its  characteristics  and  value,  de- 
pending on  its  age,  purity,  and  to  some  extent,  the  lo- 
cality in  which  it  is  produced.  It  comes  on  the  market 
mixed  mainly  with  impurities  such  as  bark,  clay  and 
other  foreign  substances  which  are  removed  by  wash- 
ing and  manipulation,  resulting  in  a  reduction  in 
wreight  of  from  10  to  50  per  cent,  the  finest  Para  rubber 
losing  about  18  per  cent. 

Rubber  used  as  insulation  is  adulterated  or  degraded 
with  various  substances,  so  that  the  compound  contains 
at  most,  only  40  per  cent  of  pure  rubber,  more  usually 
about  30  per  cent  in  the  highest  grade  insulation  down 
to  5  or  10  per  cent  in  the  poorer  rubber-covered  wires, 
with  no  rubber  in  some  cheap  insulations  which  are 
called  rubber. 


44  CABLE  INSULA  TION. 

Pure  rubber  is  valueless  for  insulating  purposes,  be- 
ing too  soft,  hydroscopic,  and  readily  oxidizable. 
When  proper  foreign  substances  to  the  extent  of  about 
60  per  cent  or  more,  including  about  3  per  cent  of  sul- 
phur, are  added  and  the  compound  vulcanized  by  heat- 
ing to  from  250  to  300  degrees  Fahr.,  the  rubber  is 
made  stable,  tough  and  durable,  its  value  as  a 
dielectric  depending  upon  the  details  of  this  treatment. 
The  exact  temperature  and  duration  of  time  necessary 
for  vulcanizing  depends  on  the  grade  of  rubber,  the  in- 
gredients used,  and  the  percentage  of  sulphur  added  to 
the  compound.  The  adulterants  most  commonly  used 
for  making  the  better  grade  rubber  compounds  are  dry 
mineral  matter  or  reclaimed  rubber;  the  composition  of 
the  particular  material  used  for  compounding,  may,  in 
many  cases,  be  left  to  the  discretion  of  the  manu- 
facturer. Proper  vulcanization  is  as  important  in  pro- 
ducing high  grade  insulation  as  the  quality  of  the  rub- 
ber used  or  the  method  of  compounding.  The  amount 
of  free  sulphur  left  in  the  compound  changes  with  oxi- 
dization. In  no  case  should  the  free  sulphur  exceed 
about  1  per  cent — the  amount  being  determinable  from 
the  acetone  extract — as  an  excess  shortens  the  life  of 
the  rubber.  Sulphur  gives  an  indication  of  the  quality 
of  the  rubber  used,  because  much  sulphur  is  required 
to  vulcanize  poor  rubber  and  a  large  amount  of  com- 
bined sulphur  may  be  taken  as  an  indication  that  it  was 
required  in  order  to  produce  vulcanization. 

The  best  rubber  comes  from  South  America,  and  is 
known  as  Para.  Weber  states  that  the  reason  for  the 


CABLE  INSULA  TION.  45 

inferiority  of  the  African  rubber  is  generally  due  to  the 
presence  of  albuminous  substances  which  are  not  re- 
moved by  washing"  and  which  result  in  a  brittle  insula- 
tion ;  he  also  states  that  light  will  oxidize  rubber,  the 
more  rapidly  the  less  the  degree  of.  vulcanization. 
While  a  compound  containing  30  'to  35  per  cent  "old 
up-river"  Para  rubber  is  generally  accepted  as  the 
requirement  for  insulation  to  be  used  in  high-tension 
work;  as  a  matter  of  fact,  it  is  almost  impossible  for 
any  chemist  to  ascertain,  after  vulcanization,  just  what 
the  constituents  of  the  insulation  may  be,  and  while 
any  number  of  tests  have  been  proposed,  it  is  much 
better  to  rely  on  the  standing  and  integrity  of  the 
manufacturer  and  his  guarantees  than  to  do  business 
with  unreliable  firms,  expecting  to  prove  from  an  exam- 
ination of  the  product  furnished  whether  or  not  they 
are  fulfilling  contract  specifications.  While  30  per  cent 
Para  seems  to  insure  high  grade  insulation,  it  is  beyond 
controversy  that  certain  compounds  containing  less 
than  30  per  cent  Para  give  most  satisfactory  results  in 
service,  although  they  fail  to  meet  some  of  the  tests 
hereafter  indicated,  as  the  requirements  for  the  best  in- 
sulation. 

High-grade  rubber  is  not  only  very  elastic  but 
possesses  great  tensile  strength.  If  over  vulcanized  it 
will  break;  if  under  vulcanized  it  is  not  elastic,  so  that 
strength  and  elasticity  are  fair  indications,  of  its  value 
as  an  insulating  material.  Those  best  informed  agree 
that  a  new  sample  of  30  to  35  per  cent  Para  compound 
properly  vulcanized,  should  be  capable  of  withstanding 


46  CABLE  INSULATION. 

a  tension  of  700  to  800  pounds  per  square  inch  before 
breaking,  and  when  stretched  from  two  to  three  times 
its  original  length  should  return  to  within  at  least  125 
per  cent  of  its  original  length,  when  at  a  temperature 
of  about  100  degrees  Fahr.  Although  insulation  con- 
taining appreciably  less  than  30  per  cent  of  Para,  with 
additional  amounts  of  cheaper  rubber,  making  a  total 
of  say,  40  to  50  per  cent,  may  pass  the  tensile  and  elas- 
tic tests  mentioned  above,  such  test  usually  indicates 
that  the  insulation  contains  only  rubber,  and  no 
shoddy  or  bituminous  products;  but  some  authorities 
claim  that  a  compound  containing  only  Para  will  have 
considerably  greater  resistance  to  puncture  than  in- 
sulations containing  the  same  proportion  of  Para  with 
a  proportion  of  inferior  rubber  in  the  materials  used 
in  compounding. 

For  the  larger  proportion  of  cables  manufactured, 
namely,  those  used  for  low  tension  work,  say  under 
5,000  volts,  30  per  cent  pure  Para  is  unnecessary,  the 
cheaper  grades,  as  high  grade  Ceylon,  Malay  or 
African  Lapori,  for  example,  giving  very  satisfactory 
results  for  this  particular  class  of  lo\v-voltage  work, 
while  even  the  rubber  produced  from  the  Mexican 
guayule  is  used  for  the  insulation  of  telephone  wires. 

The  quality  and  life  of  rubber  insulation  has  here- 
tofore generally  been  considered  as  indicated  by  the 
amount  of  resinous  or  extractive  matter  it  contained. 
A  high  percentage  of  resinous  matter,  say  15  to  20 
per  cent,  was  taken  to  indicate  a  cheaper  and  poorer 


CABLE  INSULA  TION.  47 

grade  of  rubber,  whereas  a  low  percentage,  1  to  2  per 
cent,  was  assumed  to  be  found  only  in  the  best  grade 
of  Para,  which  come  from  South  America. 

Owing  to  some  unknowrn  cause,  the  amount  of  ex- 
tractive matter  increases  largely  in  the  working  and 
mastication  of  the  gum,  there  being  still  further  in- 
crease during  vulcanization.  The  amount  of  resin- 
ous matter  is  determined  by  digesting  the  gum  in  ace- 
tone, which  dissolves  out  the  extractive  matter. 
Standard  practice  specifies  that  the  acetone  extraction 
shall  be  carried  on  for  five  hours  in  a  Soxhlet 
Extractor,  as  improved  by  Dr.  C.  O.  Weber.  It  is 
important  that  care  be  taken  in  making  the  tests, 
not  only  that  proper  conditions  should  be  observed, 
but  that  the  duration  of  the  test  is,  as  specified,  other- 
wise the  results  may  vary  widely.  As  an  example  of 
this,  it  may  be  stated  that  in  a  given  instance,  a  40 
per  cent  Para  rubber  compound,  such  as  used  by  the 
United  States  Government,  if  heated  for  a  period  of 
about  twelve  hours,  to  105  degrees  Cent.,  in  a  drying 
oven  prior  to  being  treated  with  acetone,  resulted  in 
increasing  the  weight  of  the  acetone  extract  from  2 
per  cent  to  Sj4  per  cent.  The  greater  part  of  this  in- 
crease took  place  during  the  first  few  hours  of  heat- 
ing. In  the  same  way,  the  longer  an  acetone  test  is 
continued  the  larger  the  percentage  of  extract  ob- 
tained, although  by  far  the  greater  proportion  is  given 
off  during  the  first  five  hours.  From  the  above,  the 
necessity  of  properly  conducting  and  carefully  timing 
the  length  of  the  test  will  be  recognized.  The  rubber 


48  CABLE  INSULATION. 

to  be  tested  should  first  be  dried  over  calcium  chloride 
in  a  vacuum  at  slightly  elevated  temperature,  and 
then  treated  with  acetone  in  the  extractor. 

It  should  be  understood  that  the  acetone  test  deter- 
mines the  quality  of  the  rubber  compound,  so  that  the 
usual  maximum  precentage  limit  of  5  per  cent,  for 
example,  must  be  raised  provided  it  is  intended  that 
the  manufacturer  shall  be  permitted  to  introduce  other 
substances  in  the  adulterant  used  for  compounding, 
which  in  themselves  contain  extractive  matter.  If  it 
is  clearly  stated  in  the  specifications  that  only  Para 
rubber,  to  a  definite  percentage  is  to  be  used  and 
that  the  remainder  of  the  compound  is  to  be  of  some 
other  material  than  rubber,  then  the  5  per  cent  should 
not  be  exceeded. 

There  exists  a  wide  difference  of  opinion  and  prac- 
tice as  to  the  proper  limit  of  extractive  matter  that 
should  be  permitted  in  a  high-grade  compound.  Some 
specifications*  specify  that  the  acetone  extract  should 
not  exceed  3l/>  per  cent  by  weight,  of  the  gum  in  the 
compound,  while  the  more  usual  specification  and 
that  issued  by  the  rubber  manufacturers,  sets  the 
upper  limit  of  extractive  matter,  as  5  per  cent  by 
weight,  of  the  total  compound,  that  is,  in  a  30  per 
cent  Para  compound,  the  weight  of  extractive  matter 
shall  not  exceed  about  17  per  cent  of  the  weight  of 
the  gum.  More  recent  experience  and  research  has 
shown  that  some  African  rubber  gum  may  contain  as 


*  Specifications  of  the  Railway  Signal  Association. 


CABLE  INSULATION.  49 

low  as  2  to  3  per  cent  of  extractive  matter,  while 
some  high-grade  Para,  giving  excellent  results  in 
service,  will  contain  over  4  per  cent  of  such  matter. 
With  these  wide  variations  and  with  the  knowledge 
that  by  proper  treatment  the  amount  *of  acetone  in  a 
compound  can  be  reduced  to  even  2  or  3  per  cent  if 
necessary,  the  value  of  the  acetone  test  is  being  dis- 
credited and  abandoned  by  many  engineers ;  for  ex- 
ample, the  Specifications  for  Electric  Wires  and 
Cables,,  issued  by  the  Navy  Department,*  omit  all  ace- 
tone tests  whatever,  depending  upon  other  tests  en- 
tirely to  determine  the  value  of  the  compound. 

The  introduction  into  rubber  compounds  of  waxy 
ingredients  such  as  paraffine,  for  the  purpose  of  in- 
creasing megohm  measurements,  etc.,  should  be  lim- 
ited ;  a  small  amount,  say  3  to  4  per  cent  of  the  weight 
of  the  rubber  gum  will  not  prove  injurious. 

In  America  the  rubber  compound  is  applied  to  a 
conductor  in  either  of  two  ways : 

(a)  By    passing    the    conductor    through    a    press 
similar  to  a  lead  machine  and  applying  the  compound 
in  a  plastic  state  at  relatively  high  temperature  as  a 
seamless  tube,  as  the  conductor  passes  out  of  the  ma- 
chine.    This   is   called   "spewing,"   and   is   used   more 
particularly  with  smaller  sized  conductors. 

(b)  By  applying  the   insulation   in   a  longitudinal 
strip  by  means  of  a  machine  which  folds  the  compound 

*  Dated  June  10,  1908. 


50  CABLE  INSULATION. 

around  the  conductor  and  unites  the  edges  in  a  con- 
tinuous and  almost  invisible  seam.  As  judged  from 
practical  results,  the  strip  insulation  seems  about  as 
satisfactory  and  reliable  as  the  seamless  insulation,  and 
it  has  the  further  advantage  of  keeping  the  conductor 
properly  centered  and  having  imperfections  in  one 
layer  covered  by  additional  layers. 

In  Europe  the  insulation  of  conductors  by  winding 
with  rubber  tape  has  been  successfully  accomplished. 
This  method  should  be  expected  to  result  in  a  more 
uniform  dielectric  capable  of  withstanding  greater 
potential  stress,  for  a  given  thickness,  than  when  ap- 
plied by  "spewing"  or  in  strips. 

With  any  method  of  applying  the  compound,  a  braid 
or  tape  over  all,  is  employed  to  better  hold  the  com- 
pound in  position  and  prevent  its  swelling  and  becom- 
ing porous  during  vulcanization  ;  such  tape  has  no  par- 
ticular value  as  a  dielectric. 

Rubber  insulated  underground  cables  are  usually 
covered  with  a  lead  sheath  both  for  mechanical  protec- 
tion and  to  guard  against  attacks  from  oils,  acids  or 
oxidization.  The  substitution  of  a  fibrous  covering 
served  with  a  bituminous  compound  or  something  of 
that  sort,  has  been  attempted  in  place  of  the  lead  sheath 
and  is  said  to  be  found  satisfactory  under  some  condi- 
tions, particularly  where  electrolysis  cannot  be  avoided, 
although  such  substitution  is  ordinarily  based  pri- 
marily on  considerations  of  cost. 

When  completed  the  rubber  insulated  cable  is  the 
most  flexible  of  all,  and  should  be  capable  of  being 


CABLE  INSULATION.  51 

bent  on  a  radius  equal  to  five  times  its  diameter,  bent 
similarly  in  a  reverse  direction ;  have  the  process  re- 
peated three  times  and  then  withstand  puncture  and 

ohmic  tests  hereinafter  specified. 

• 

Paper  Insulation.  Paper  insulation  is  made  by  tap- 
ing paper  ribbon  about  a  conductor  in  successive  layers 
until  the  required  thickness  is  obtained.  The  cable  is 
then  dessicated  by  baking,  or  more  satisfactorily  by 
giving  it  a  preliminary  drying  in  air  and  placing 
in  a  vacuum,  and  immediately  immersed  in  a  bath  of 
oily  insulating  resinous  compound,  at  a  temperature  of 
not  less  than  120  degrees  Cent.  (250  degrees  Fahr.), 
until  thoroughly  saturated  ;  the  whole  is  then  promptly 
enclosed  with  a  lead  sheath,  which  is  necessary  to  ex- 
clude moisture,  and  at  the  same  time,  holds  the  insulat- 
ing compound  in  position.  The  value  of  the  insulation 
as  a  dielectric  depends  on  the  quality  of  the  paper  and 
the  compound. 

The  best  paper  is  that  made  from  Manilla  fibre,  pri- 
marily because  of  its  mechanical  strength.  The  paper 
should  show  uniform  texture  when  held  to  the  light, 
be  free  from  coarse  or  metallic  particles,  or  pin  holes, 
and  should  show  no  trace  of  chlorine  or  other  residual 
chemicals,  or  be  loaded  with  low  grade  material.  Strips 
of  paper  five-thousandths  of  an  inch  in  thickness,  and 
one  inch  wide,  after  being  impregnated  with  the  in- 
sulating compound  to  be  used,  should  sustain  without 
breaking,  a  load  of  40  pounds.  The  thickness  of  paper 
ordinarily  used  is  from  five  to  six-thousandths  of  an 


5  2  CABLE  INSULA  TION. 

inch,  with  a  tendency  toward  thinner  papers  for  the 
higher  voltages.  The  width  of  the  paper  ribbon  em- 
ployed varies  from  one  to  two  and  a  half  inches,  the 
widest  ribbon  being  used  on  the  conductors  of  large 
diameter. 

Rosin  oil,  which  is  the  diluent  and  chief  ingredient 
of  the  fluids,  used  for  impregnating  paper  insulation, 
is  obtained  from  the  distillation  of  rosin  gum.  Rosin 
comes  from  oleo  turpentine,  which  is  exuded  by  the 
long-leaf  pine  or  coniferous  trees.  Rosin  produces 
rosin  oil  and  pitch ;  the  former  is  distilled  a  second 
time  producing  what  is  known  as  "second  oil,"  which, 
more  or  less  treated  or  refined,  is  the  impregnating 
fluid  used  as  the  principal  dielectric  in  paper-insulated 
cables.  The  method  of  preparing  the  rosin  oil  for 
impregnating,  varies  with  the  different  manufacturers 
in  accordance  with  their  particular  formulae — which 
like  those  relating  to  the  ingredients  of  rubber  com- 
pounds, are  guarded  as  "State  Secrets" — and  make  the 
chief  difference  in  the  quality  of  paper  insulation. 

Lack  of  uniformity  in  commercial  rosin  oil,  its  lia- 
bility to  contain  moisture  and  deleterious  substances, 
necessitate  the  greatest  care  in  the  proper  prepara- 
tion of  rosin  oil  for  insulating  purposes.  A.  Bartoli* 
gives  the  relative  value  of  insulating  oils,  and  it  is 
noteworthy  that  those  which  are  the  more  capable  of 
being  oxidized  are  the  less  valuable  as  dielectrics, 
which  would  indicate  a  departure  from  the  present 
use  of  rosin  oil  in  its  usual  unoxidized  condition. 


*  L.  L.  Nuovocimento,  1890,  vol.  XXVIII.  page  25. 


CABLE  INSULATION.  53 

In  the  application  of  rosin  oil  to  paper,  the  oil 
— abietic  anhydride,  C44  H62  O4, — seems  to  soak  into 
the  paper  leaving  the  rosin  largely  on  the  outside. 
The  insulation  shows  the  highest  puncture  tests  when 
the  pores  of  the  paper  are  filled  with  .oil,  which  may 
take  many  hours  or  even  days,  at  low  temperature, 
to  accomplish,  where  the  impregnation  is  made 
through  many  layers,  of  paper. 

The  use  of  too  viscuous  oil  results  in  the  absorption 
of  the  diluent  by  the  paper  leaving  the  rosin  "high 
and  dry,"  resulting  in  a  non-flexible  and  hard  cable. 
Recently,  the  advantage  of  using  a  more  fluid  oil  has 
been  recognized,  which,  wrhile  reducing  the  megohm 
measurements,  results  in  a  cable  that  will  withstand 
satisfactorily  high  puncture  tests,  and  at  the  same  time, 
make  it  more  flexible  and  thus  largely  avoid  the 
difficulties  that  have  heretofore  been  encountered  in 
handling  paper  cables,  namely,  their  liability  to  split 
or  crack  when  bent,  particularly  in  cold  weather.  In- 
vestigation and  experiment  has  recently  produced  a 
very  much  improved  quality  of  rosin  oil,  which  does 
not  become  viscous  even  at  zero  degrees  Fahrenheit,  so 
that  one  very  practicable  objection  to  paper  cables, 
their  lack  of  flexibility,  is  now  likely  to  be  removed. 
With  all  cables,  however,  it  is  just  as  well  to  keep  them 
in  a  warm  room  for  some  hours  if  they  are  to  be 
installed  when  the  temperature  is  below  freezing. 

As  long  as  the  paper  insulation  of  cables  can  be  kept 
intact  within  their  lead  sheaths,  they  are  found  to 
give  most  excellent  satisfaction ;  but  if  by  reason  of 


54  CABLE  INSULATION. 

defects  in  manufacture,  electrolysis  or  damage,  the 
sheaths  are  punctured  so  that  water,  or  even  water 
vapor,  can  gain  access  to  the  dielectric,  the  breaking 
down  of  the  insulation  is  a  question  of  minutes,  or  at 
most,  hours. 

At  the  time  of  manufacture,  the  lead  sheathing  of 
paper  cables  is  continued  so  as  to  completely  enclose 
and  protect  the  ends  of  the  insulated  conductors  with 
lead,  to  keep  out  moisture.  The  lead  sheath  should 
never  be  stripped  off  the  ends  of  the  cable  until  every- 
thing is  prepared  for  making  a  prompt  and  dry  joint, 
or  inserting  in  an  "end  bell"  for  making  a  terminal. 
The  stiffness  of  paper  cables  is  related  to  their  tem- 
perature and  the  quality  of  the  impregnating  fluid;  but 
with  the  use  of  the  best  oils,  a  cable  should  be  capable 
of  being  bent  back  and  forth  three  times,  on  a  radius 
of  eight  times  its  diameter,  even  at  a  temperature  of 
freezing,  and  then  withstand  the  regular  puncture  and 
ohmic  tests. 

On  account  of  their  relatively  low  first  cost,  paper  in- 
sulated cables  are  being  more  and  more  used  for  all 
services — even  submarine — and  are  proving  success- 
ful, despite  their  inherent  limitations.  There  are  more 
miles  of  high  tension  cables  in  use  insulated  with  pa- 
per than  with  all  other  insulations  combined. 

Cambric  Insulation.  A  recently  developed  dielectric 
for  insulating  high-tension  cables  is  varnished  muslin 
or  cotton  fabric  usually  called  cambric.  The  muslin  is 
coated  on  both  sides  with  several  separate  films  of  insu- 


CABLE  INSULATION.  55 

lating  varnish,  or  in  some  cases,  linseed  oil  com- 
pounded with  some  paraffine  or  ozokerite,  or  even 
rosin.  The  coated  material  is  then  cut  into  strips  mak- 
ing ribbon  which  is  wound  spirally  about  the  conduc- 
tor in  layers  to  any  desired  thickness;  between  the 
wrappings  is  applied  a  thin  layer  of  viscuous  adhesive 
compound  which  prevents  the  unwrapping  of  the  tape 
when  cut,  largely  precludes  the  absorption  of  moisture, 
and  increases  the  flexibility  by  permitting  the  layers 
of  cambric  to  slide  upon  one  another.  More  usually 
a  thin  layer  of  pure  rubber,  or  in  some  cases,  treated 
paper  or  cloth,  is  first  applied  to  the  conductor  before 
the  cambric  insulation  is  put  on,  in  order  to  prevent 
the  varnish  from  attacking  the  copper,  and  in  the  case 
of  the  rubber,  to  secure  a  dielectric  or  high  resistance 
next  the  conductor.  Asbestos  has  also  been  used  as 
a  separator,  with  the  idea,  among  others,  of  permitting 
greater  heating,  that  is,  greater  carrying  capacity, 
without  injury  to  the  varnished  cambric. 

The  application  of  the  dielectric  by  taping,  with  the 
use  of  a  filling  compound,  as  is  the  case  with  paper 
insulation,  should  result  in  avoiding  such  defects  in 
the  dielectric  as  the  formation  of  air  pockets  and  decen- 
tralization of  the  conductor,  that  are  possible  with 
"spewed"  rubber  insulation.  The  splicing  of  cambric 
cables  is  more  simple  than  with  paper  insulation,  as 
moisture  is  not  as  readily  absorbed  nor  is  the  cambric 
attacked  by  mineral  oils,  making  it  particularly  con- 
venient for  connecting  into  apparatus  submerged  in 
oil,  as  switches,  transformers,  etc.  For  station  wiring, 


56  CABLE  INSULATION. 

varnished  cambric  can  be  installed  without  a  metallic 
sheath  and  does  not  require  end  bells,  for  which  service 
it  is  usually  finished  with  a  tape  and  asbestos  braid. 

Cambric  insulated  high-tension  cables  should  not  be 
continuously  operated  at  higher  temperatures  than 
rubber,  preferably  not  above  about  65  degrees  Cent., 
whereas  paper  insulated  high-tension  cables  may  be 
safely  operated  at  about  80  degrees  Cent.  Aside  from 
somewhat  increased  flexibility  and  less  liability  of  in- 
jury from  moisture  in  case  of  injury  to  the  lead 
sheath,  or  where  it  is  desired  to  install  cables  without 
a  lead  sheath,  as  in  a  power  station,  cambric  insulation 
seems  to  offer  no  very  especial  advantages  over  paper 
insulation,  particularly  at  existing  prices,  as  the  paper 
cables  are  appreciably  less  expensive  than  those  with 
cambric  insulation.  The  usual  practical  advantage  ad- 
vanced for  cambric  insulation,  as  against  rubber,  seems 
to  be  that  of  cost ;  but,  on  the  other  hand,  charring  be- 
tween the  layers  of  the  cambric  has  been  observed,  due 
possibly  to  air  bubbles ;  and  the  question  has  also  been 
raised  whether  the  ageing  and  drying  out  of  the  var- 
nish will  not  cause  the  insulation  to  become  friable  and 
deteriorate,  particularly  if  operated  at  relatively  high 
temperatures. 

Shellaced  cambric  insulation  is  considerably  more 
pliable  than  paper  and  the  cable  complete  should  with- 
stand the  puncture  tests  given  on  a  later  page  after 
being  bent  three  times  in  opposite  directions  on  a 
radius  equal  to  six  times  its  diameter. 


CABLE  INSULATION.  57 

Dielectric  Stresses.  The  dielectric  strength  of  rub- 
ber is  much  higher  than  that  of  treated  paper  or  var- 
nished cambric,  being  as  a  maximum  as  high  as  20,000 
volts  per  millimeter  of  thickness  in  thin  sheets,  whereas 
the  same  thickness  of  treated  paper  will  not  withstand 
more  than  from  one-half  to  two-thirds  this  potential, 
so  that  unless  some  other  materials  are  found,  or 
further  improvement  be  made  in  paper  insulation, 
which  seems  possible,  it  is  likely  that  rubber  must  be 
used,  at  least  in  part,  on  cables,  designed  for  the  highest 
potentials,  in  order  that  the  completed  cable  may  not 
become  so  great  in  diameter  as  to  be  cumbersome  and 
impracticable  to  handle.  It  was  early  appreciated  that 
doubling  or  tripling  the  thickness  of  a  given  insulation 
did  not  increase  its  ability  to  stand  up  under  applied 
electrical  stresses,  in  anything  like  the  same  ratio.  It 
was  found,  with  an  insulation  of  homogeneous  mate- 
rial, that  the  fall  of  potential  through  the  insulation, 
from  the  conductor  to  the  lead  sheath,  was  not  uniform 
but  increased  very  much  more  rapidly  nearest  the  con- 
ductor, being  for  a  certain  insulation,  for  example, 
5,000  volts  per  millimeter  of  insulation  next  the  con- 
ductor and  only  1,000  volts  for  the  same  thickness  next 
the  sheath.  Without  more  fully  considering  what  may 
be  the  fall  of  potential  along  the  radii,  from  the  sur- 
face of  the  conductor  to  the  sheath,  or  the  complex 
formulae  by  which  these  values  may  be  calculated,  for 
various  dielectrics,  it  may  be  said  that  both  theory  and 
experiment  prove  the  fact;  and,  furthermore,  that  the 
rate  of  fall  of  potential  varies  with  different  materials, 


58  CABLE  IXSULATIOX. 

depending  upon  their  various  specific  inductive  capaci- 
ties. Knowledge  of  these  conditions  led  an  English- 
man, Mr.  M.  O'Gorman,  and  an  Italian,  Mr.  E.  Jona, 
about  the  same  time,  to  suggest  equalizing  the  fall  of 
potential  so  as  to  secure  a  uniform  or  practically 
uniform  ''potential  gradient"  throughout  the  insulation 
either  by  impregnating  the  insulating  material  to 
different  extents  depending  on  its  distance  from  the 
conductor,  or  by  applying  successive  layers  of  insula- 
tion each  made  up  to  have  different  inductive  capacities 
with  the  layers  arranged  so  that  those  of  material  with 
the  highest  capacity  should  be  nearest  the  conductor. 
This  arrangement  of  insulating  material  causes  the 
outer  layers  to  support  approximately  the  same  strains 
per  unit  of  thickness  as  the  inside  layers;  and  hence, 
the  total  stress  due  to  the  potential  of  the  conductor  is 
supported  by  a  wall  having  a  total  thickness  of  insula- 
tion very  much  less  than  if  homogeneous.  Theo- 
retically, the  insulating  material  should  vary  gradually 
instead  of  by  layers ;  but  this,  of  course,  is  imprac- 
ticable, so  that  the  fall  of  potential  from  conductor  to 
sheath  proceeds  by  a  series  of  small  steps  instead  of  in 
a  smooth  curve. 

Experiment  with  high  potentials  seems  to  have 
demonstrated  that  the  distribution  of  stress  in  solid 
dielectrics,  such  as  paper  or  rubber,  is  very  similar  to 
that  wrhich  we  know  occurs  in  air.  About  conductors 
of  small  diameter  air  apparently  breaks  down,  resulting 
in  a  conducting  medium  made  up  of  the  solid  conductor 
and  air,  which  is  considerably  larger  in  diameter  than 


CABLE  INSULA  TION.  5  9 

the  solid  material.  It  is  probable  that  similar  action 
takes  place  with  the  insulation  about  conductors  of 
small  diameter,  so  that  the  dielectric  itself,  for  a  small 
distance  from  the  conductor,  breaks  down  and  be- 
comes also  a  conducting  medium.  In  any  case,  it  is 
clear  that  insulation  of  a  given  character  about  a  con- 
ductor of  large  diameter  will  sustain  a  considerably 
higher  potential  before  puncture,  than  the  same  insula- 
tion about  a  small  conductor.  As  a  result  of  experi- 
ments made  by  him,  Mr.  Jona  concludes  that  by 
sheathing  a  copper  conductor  in  lead,  thus  both  in- 
creasing its  diameter  and  affording  an  absolutely 
smooth  and  cylindrical  exterior,  there  may  be  produced 
"a  diminution  in  the  potential  gradient  in  the  very 
first  stratum  of  dielectric  of  something  like  20  to  30  per 
cent  or  even  more,"  and  he  has  so  sheathed  with  lead 
high-tension  cables  made  under  his  direction. 

Graded  Insulation.  The  theory  of  applying  layers 
of  insulating  material  having  different  capacities  has 
been  carried  out  in  practice  and  the  value  of  "graded" 
cables  for  high  potentials  successfully  demonstrated. 
For  example,  there  was  shown  at  the  1906  Milan  Ex- 
hibition, such  cable,  having  a  total  thickness  of  insula- 
tion of  only  14.5  m.m.,  though  designed  for  a  normal 
working  pressure  of  100,000  volts,  and  at  present,  there 
arc  installed  across  the  Lake  of  Garda,  Italy,  single- 
conductor  "graded"  cables  operating  at  13,000  volts. 
These  cables,  are  insulated  by  several  layers  of  vulcan- 
ized india  rubber  to  a  total  thickness  of  5.5  mm.  Out- 
side the  rubber  is  a  coating  of  1.2  mm.  of  gutta  percha 


60  CABLE  INSULA  TION. 

to  further  insure  imperviousness.  This  is  covered  with 
"tanned  jute"  and  armored  with  No.  18  steel  wire 
3  m.m.  in  diameter.  As  three  of  these  cables  are 
required  for  three-phase  operation  an  interesting  plan 
was  adopted  in  order  to  avoid  undue  self-induction; 
each  of  the  steel  wires  used  in  armoring  was  wrapt 
with  tarred  hemp  before  being  wound  around  the 
insulated  conductor.  The  result  of  this  experiment 
seems  to  be  satisfactory,  as  the  drop  of  pressure  due  to 
self-induction  is  reported  to  have  been  reduced  to  the 
same  amount  as  the  drop  due  to  the  ohmic  resistance. 
Connecting  the  generating  station  and  transformer 
house  of  the  Ontario  Power  Company  at  Niagara  Falls, 
are  some  high  voltage  "graded"  cables. 

Variation  in  the  capacity  of  rubber  used  for  "grad- 
ed" cable  is  obtained  by  "loading"  it  with  other  sub- 
stances such  as  talc,  zinc,  etc.,  while  the  capacity  of 
paper  may  be  similarly  varied  by  changing  the  quality 
of  the  'paper  or  the  process  of  impregnating.  The 
process  used  at  present  for  impregnating  cables  has  the 
effect  of  sometimes  giving  the  greater  dielectric 
strength  and  capacity  where  they  are  not  wanted, 
namely,  in  the  outer  layers  of  the  insulating  material. 
This  is  due  to  the  fact  that  the  liquid  used  for  impreg- 
nating more  easily  reaches  and  solidly  fills  the  outer 
portions  of  the  dielectric.  As  often  manufactured, 
rubber  cables  are  subject  to  the  same  fault;  because 
pure  rubber,  which  is  of  the  lowest  specific  capacity, 
is  placed  next  the  conductor,  the  tougher,  degraded  or 
vulcanized  rubber  of  greater  capacity  being  used  for 


CABLE  INSULATION.  61 

the  outer  layers.  While  the  unequal  distribution  of 
dielectric  strength  is  of  little  importance  in  itself,  there 
is  greater  danger  of  a  breakdown  than  if  the  insulation 
were  homogeneous  throughout,  due  to  the  increased 
capacity  created  in  the  outer  layers. 

Composite  Construction.  Not  with  a  view  to  ob- 
taining the  results  to  be  secured  by  "grading"  the  in- 
sulation but  primarily  for  the  purpose  of  reducing  the 
cost,  cables  have  recently  been  made  with  rubber  and 
paper,  or  rubber  and  cambric  insulation  combined. 
By  using  rubber  next  to  the  conductor  and  paper  or 
cambric  outside  the  rubber,  the  more  expensive  and 
better  insulation  is  distributed  where  its  greater 
strength  is  most  advantageously  used.  Attempts  have 
been  made  to  enclose  paper  insulation  with  a  light 
jacket  of  rubber  as  a  protection  against  moisture ;  but 
owing  to  the  difficulty  of  vulcanizing  the  rubber  with- 
out injuring  the  paper,  such  results  have  met  with  but 
little  success. 

Where  two  or  three-phase  currents  are  employed  for 
high-tension  work,  the  several  underground  conductors 
required  for  such  a  circuit  are  usually  separately  in- 
sulated, laid  up  with  jute  and  then  the  whole  enclosed 
in  a  "jacket"  or  "belt"  of  insulating  material,  which 
further  insulates,  to  ground,  economizes  space  and 
insulation  and  especially  protects  mechanically.  For 
full  working  potential  between  conductors  and  ground, 
the  "jacket"  or  "belt"  is,  particularly  with  paper  and 
cambric,  usually  equal  in  thickness  to  the  insulation 
about  each  conductor;  in  case  of  star-connected  circuits 


62  CABLE  INSULATION. 

with  grounded  neutrals,  the  insulation  between  con- 
ductor and  ground  need  be,  theoretically,  but  six- 
tenths  that  between  conductors,  practically,  however, 
it  is  made  somewhat  heavier  than  theory  would  re- 
quire. 

At  present  there  seems  to  exist  a  well-founded  feel- 
ing that  too  much  money  has  been  expended  in,  and 
too  high  an  electrical  value  placed  on  the  "jacket'*  or 
"belt,"  usually  employed  with  high-voltage  cables.  In 
considering  whether  or  not  it  is  desirable  to  use  part 
of  the  insulation  of  such  a  cable  in  a  "jacket"  or  "belt", 
or  whether  the  same  expenditure  for  insulation  could 
be  better  made  in  thickening  the  insulation  about 
each  conductor,  it  should  be  borne  in  mind  that  if 
the  "belt"  is  injured — as  will  usually  be  the  case  if  a 
breakdown  occurs — its  value  is  reduced  to  little  or 
nothing,  as  supplementing  the  insulation  about  the 
two  other  conductors,  which  may  be  uninjured.  This 
reasoning  relates  to  electrical  considerations  and  does 
not  include  the  mechanical  advantages  obtained  by 
the  application  of  a  second  separate  and  distinct  layer 
of  insulation  which  affords  a  smooth,  even  surface  for 
the  application  of  the  lead  sheath,  and  withal  makes 
the  cable  more  flexible.  It  would  seem  as  if  a  lighter 
belt  and  heavier  insulation  about  each  conductor 
would  be  more  advantageous  than  the  present  gen- 
eral practice  of  making  the  belt  and  the  insulation 
about  each  conductor  of  the  same  thickness  per  thou- 
sand volts  of  potential  stress. 

Concentric  cables  consisting  of  a  rod,  insulated,  and 


CABLE  1XSULA  TION.  63 

inserted  in  one  or  two  metal  tubes,  as  the  second  or 
third  conductor,  were  early  employed,  particularly 
abroad ;  but  have  hardly  demonstrated  their  claims  to 
superiority;  their  use  is  being  restrained,  in  Germany, 
for  example,  being'  prohibited  for  voltages  over  3,000. 
For  low  voltage  work,  concentric  cables  offer  some 
advantages  which  are  extending"  the  use  of  such  cables 
in  America.  On  account  of  the  increased  thickness 
of  insulation  required  with  higher  potentials,  say  from 
50,000  volts  upward,  single  conductor  cables  will  prob- 
ably be  necessary  for  such  potentials,  at  least  when 
they  are  to  be  much  handled  or  drawn  in  ducts. 

Thickness  of  Commercial  Insulations.  Various 
formulae  have  been  suggested  by  which  to  determine 
the  proper  thickness  of  the  different  insulations  to  use 
for  a  given  potential.  Such  formulae  usually  contain 
empirical  constants,  the  value  of  which  largely  depends 
on  a  personal  equation.  The  errors  caused  by  the 
practical  difficulties  of  manufacture,  such  as  eccentric 
placing  of  the  insulation  about  the  wire,  unevenness  of 
application,  imperfections  in  the  dielectric,  mechanical 
considerations  of  strength,  make  tables  of  insulation 
required  for  different  voltages  and  sizes  of  conductors, 
much  more  valuable  and  reliable,  than  formulae,  as  the 
former  are  based  on  practical  experience,  tests  and 
guarantees  that  manufacturers  are  willing  to  stand 
back  of. 

In  determining  the  thickness  of  insulation  of  high- 
tension  cables,  whether  from  the  standpoint  of  theo- 
retical design  or  consideration  of  actual  installations,  it 


64  -  CABLE  INSULATION. 

must  be  borne  in  mind  that  quantity  gives  no  indica- 
tion of  the  quality  of  dielectrics.  Furthermore,  the 
normal  voltage  at  which  a  cable  may  be  expected  to  be 
operated  gives  little  indication  of  the  monetary  or  di- 
electric values  of  the  insulation  used ;  these  values  are 
determined  rather  by  the  factor  of  safety  employed  and 
the  breakdown  or  puncture  tests  which  the  cables  must 
pass.  The  superiority  of  a  given  character  of  insula- 
tion furnished  by  one  manufacturer  as  compared  with 
that  of  another  manufacturer  for  a  given  service,  of 
necessity  compels  relegating  to  a  secondary  considera- 
tion the  question  of  mere  thickness  of  a  dielectric.  As 
one  manufacturer  has  expressed  it,  "puncture  tests 
rather  than  working  voltage,  or  thickness  of  insula- 
tion, is  what  we  want  specified."  Nevertheless,  the 
following  information  is  here  submitted,  not  as  indicat- 
ing the  minimum  limiting  thickness  of  the  best  grade 
of  insulation  for  the  voltages  specified,  but  as  showing 
in  a  general  way  what  some  representative  manu- 
factures are  offering,  and  as  a  conservative  guide  to 
what  can  reasonably  be  asked  and  obtained. 

Mr.  H.  G.  Stott  states  that  from  his  experience, 
paper  insulation  for  3,000  volts  on  wires  from  No.  6  to 
No.  00  B.  &  S.,  inclusive,  should  be  5/32  of  an  inch 
thick,  and  for  larger  sizes  up  to  300,000  c.  m.,  6/32  of 
an  inch  thick  with  an  increase  of  1/32  inch  for  each 
1,000  volts  up  to  11,000  volts  and  after  that  1/64  inch 
additional  insulation  for  each  1,000  volts.  For  35 
per  cent  Para  rubber  compound  or  varnished  cambric, 
he  states  that  it  is  only  necessary  to  add  1/64  inch 


CABLE  INSULATION.  65 

additional  insulation  for  each  1,000  volts  above  3,000 
until  25,000  volts  is.  reached. 

The  General  Electric  Company,  Schenectady,  N.  Y., 
for  three-conductor  stranded,  varnished-cambric  insu- 
lated, leaded  cables,  recommending  the  -same  thickness 
of  insulation  about  each  conductor  as  in  the  jacket, 
give  the  following  figures: 

TABLE    II. 

THICKNESS  OF  CAMBRIC  INSULATION. 
(G.  E.  CO.) 


Normal 
Working  Voltage 

Insulation  about 
each  Conductor 

Insulation  about 
three   Conductors 

7,OOO 

4/32    inch 

4/32    inch 

10,000 

5/32      " 

5/32       " 

13,000 

6/32      " 

6/32       " 

17,000 

7/32      " 

7/32       " 

20,000 

8/32      " 

8/32       " 

23,000 

17/64      " 

17/64      " 

25,000 

18/64 

18/64      " 

^General  Electric  Co.     Bulletin  Mo.  4591. 


The  Safety  Insulated  Wire  &  Cable  Company,  New 
York,  specify  the  following  thicknesses  for  rubber  (30 
per  cent  Para),  and  paper  insulated  cables,  they  do  not 
furnish  varnished-cambric  insulation. 

It  will  be  noted  that  no  jacket  is  provided  with  the 
rubber-insulated  cables  intended  for  use  at  the  lower 


66  CABLE  INSULATION. 

potentials,  this  is  due  to  the  fact  that  a  thin  rubber 
jacket  will  be  relatively  largely  reduced  in  thickness  by 
the  pressure  from  the  insulated  conductors,  as  it  seems 
impossible,  practically,  to  maintain  uniform  pressure  of 
the  jute  filling1  and  the  conductors  against  the  jacket. 

TABLE   III. 

THICKNESS  OF  RUBBER  AND  PAPER  INSULATION. 
(S.  I.  W.  &  C.  CO.) 


RUBBER  INSULATION  PAPER  INSULATION 

Normal 
Working 


Voltage 

About 

each     About  three 

About  each 

About 

three 

t> 

Conductor       Conductors 

Conductor 

Conductors 

^  ,000 

5/32 

inch        None 

4/32  inch 

4/32 

inch 

7,000 

7/32 

None 

5'3*     " 

5/32 

t  I 

10,000 

5/32 

5/32  inch 

6/32      " 

5/32 

1  ' 

13,000 

7/32 

"        5/32     " 

7/32     " 

6/32 

(.  i 

17,000 

8/32 

5/32     " 

7/32      " 

•1  '  "»-> 
7/32 

i  ' 

20,000 

9/32 

6/32     " 

8/32     " 

8/32 

•' 

25,OOO 

10/32 

"        7/32     " 

10/32 

10/32 

.  i 

30,000 

12/32 

"       10/32     " 

12/32      " 

12/32 

.  i 

Pirelli  and  Company,  Milan,  Italy,  usually  employ 
impregnated  paper  for  cables  up  to  20,000  volts ;  for 
higher  pressures  they  employ  their  own  special  system 
of  india-rubber  and  paper  insulation.  As  indicating  in 
a  very  general  way  their  practice,  the  following  figures 
are  given  : 


CABLE  INSULATION. 

TABLE   IV. 

THICKNESS  OF  PAPER  INSULATION. 
(P.  &  CO.) 


Normal 
Working  Voltage 


IO,OOO 
l6,OOO 
2O,OOO 


Total  'thickness  of 
Insulation 


.27   inch 

.38   " 
•50   " 


The  British  Insulated  &  Helshy  Cables,  Ltd.,  Pres- 
cot,  England,  gives  the  same  thickness  of  insulation  on 
three-conductor  cables  that  is  specified  by  the  Engi- 
neering Standards  Committee,  as  follows,  for  medium 
size  conductors  : 

TABLE  V. 

THICKNESS  OF  RUBBER  AND  PAPER  INSULATION. 
(B.  I.  &  H.  C.,  LTD.) 


N  ormal 
Working 
Voltage 

RUBBER 

INSULATION 

PAPER  INSULATION 

About  each 
Conductor 

Jacket 
about  star- 
connected 
<and  grounded 
Conductors 

About  each 
Conductor 

Belt 

about  star- 
connected 
and  grounded 
Conductors 

6,600 
11,000 

.21  inch 

•30  " 

.  10  inch 
.11     " 

1 

24  inch 

.36      " 

.  1  8  inch 

.24 

Most  of  the  above  tables  are  based  on  full  working 
potentials  between  conductors  or  between  conductor 


68  CABLE  INSULATION. 

and  ground.  In  case  the  three  conductors  are  used  for 
star-connected  circuits  with  grounded  neutrals,  the 
thickness  of  insulation  between  a  conductor  and 
ground  need  be  but  6/10  of  that  between  conductors, 
but  in  practice  it  is  made  somewhat  thicker  than 
theory  dictates.  This  relation  of  voltage  and  insula- 
tion should  be  borne  in  mind  when  testing  and  all 
tests  on  the  cable  should  be  properly  proportioned  to 
the  thickness  of  the  insulation. 

Joints.  The  purpose  of  a  paragraph  on  cable  joints 
is  not  to  teach  splicing  to  those  unacquainted  with  the 
methods  employed,  but  rather  for  the  purpose  of  em- 
phasizing the  importance  of  this  part  of  cable  installa- 
tion. Reference  has  already  been  made  to  the  gen- 
erally admitted  fact  that  joints  are  the  weakest  points 
in  high-tension  cables.  This  is  so  not  because  the 
joints  necessarily  need  be  \veak,  but  because  proper 
attention  and  care  has  not  been  exercised  in  making 
them.  More  breakdowns  in  cable  operation  have 
probably  resulted  from  defective  joints  than  from  all 
other  internal  causes.  Careful  and  competent  work 
in  making  joints  at  the  time  of  installation  will  later 
avoid  much  worry,  inconvenience  and  monetary  loss. 
Except  for  the  largest  companies,  which  can  afford  to 
maintain  regularly  in  their  employ  high  grade,  experi- 
enced cable  workmen,  it  is  advisable  for  all  purchasers 
to  include  in  their  contracts  with  the  manufacturers, 
the  drawing-in  and  jointing  of  cables  complete. 

Various  types  of  tape  and  compound  are  employed 


CABLE  INSULATION.  69 

for  making  cable  joints,  depending  on  the  insulation 
used.  For  rubber  insulated  cables  usually  a  layer  of 
pure  rubber  is  applied  and  then  compounded  tape  is 
used,  which  must  be  vulcanized,  after  application, 
by  immersion  in  a  bath  of  suitable  hot  compound  or 
by  mearfs  of  a  torch  used  only  in  the  hands  of  an 
expert,  as  undue  heat  applied  to  rubber  tape  will  in- 
jure it.  Paper  and  cambric-insulated  cables  have  their 
joints  wrapped  with  paper  or  cotton  tape,  which  may 
well  be  kept  in  the  hot  compound,  which  later  will  be 
used  for  filling  the  lead  sleeve.  All  joint  wrapping 
material  left  exposed  to  the  air  or  moisture  deteriorates 
and  should  be  carefully  protected.  Even  the  moisture 
from  the  hands  of  the  workmen  has  been  known  to  be 
sufficient  to  destroy  an  otherwise  perfect  splicing  job. 
No  acids  should  be  used  as  a  flux  in  soldering,  as  it  is 
liable  to  injure  the  insulation. 

The  secret  in  making  a  perfect  joint,  provided  proper 
materials  are  furnished,  is 

(a)  Exclude  all  moisture. 

(b)  Make  the  wrappings  as  tight  as  possible,  to  ex 
elude  air. 

(c)  Have  the  layers  of  tape  overlap  and  adhere  uni- 
formly. 

(d)  Be  certain  the  compound  is  sufficiently  hot,  be- 
fore pouring. 

The  insulation  in  a  joint  should  be  made  somewhat 
thicker  than  that  about  the  conductor.  A  com- 
mon rule  is  to  make  the  insulation  at  the  splice  150 
per  cent  of  that  about  the  conductor. 


70  CABLE  INSULA  TION. 

For  paper-insulated  cables,  a  special  paper  tube  has 
been  brought  out  large  enough  to  slip  over  the  insula- 
tion about  a  conductor,  so  that  after  the  conductors  are 
spliced  and  taped  the  tube  is  drawn  over  the  splice  and 
further  tape  added ;  and  from  the  records  made,  this 
method  of  insulating  a  joint  seems  to  give  very  satis- 
factory results,  the  advantage  of  having  part  of  the 
insulation  about  the  joint  made  up  in  a  uniform  and 
perfect  manner  in  a  factory,  will  be  recognized. 

The  method  of  making  joints,  which  has  resulted  in 
the  excellent  record  of  paper  cables,  referred  to  on  page 
35,  may  be  interesting,  as  no  special  tools  or  particu- 
larly skilled  labor  are  required.  The  outside  paper  belt 
of  the  cable  is  first  removed  and  the  jute  filler  turned 
back  and  tied  down  out  of  the  way.  The  ends  of  the 
conductors  are  bared  and  joined  by  metal  sleeves 
properly  sweated  on.  An  insulating  compound,  manu- 
factured in  Chicago,  heated  to  about  150  degrees  Cent. 
(300  degrees  Fahr.),  is  then  poured  over  the  conduct- 
ors to  drive  out  all  moisture  which  may  be  present, 
after  which  strips  of  treated  paper  tape  are  wrapped  as 
tightly  as  possible  about  the  metal  joints  to  a  thickness 
appreciably  greater  than  that  of  the  insulation  about 
the  conductor,  paper  tubes  sometimes  being  used 
in  connection  with  tape;  the  heated  compound  is  also 
poured  over  the  joint  during  the  wrapping  to  keep  out 
moisture.  AYhen  the  three  joints  are  completely  taped, 
the  filler  is  turned  back  to  fill  the  hollows  between  the 
conductors;  the  whole  being  further  wrapped  with 
paper  tape  to  somewhat  greater  thickness  than  the 


CABLE  IXSULA  TIOX.  7 1 

jacket  insulation.  I  loles  are  punched  through  the  outer 
wrapping  near  each  end  to  permit  the  escape  of  air 
and  the  admission  of  the  compound.  The  lead  sleeve 
is  then  slipped  over  the  insulated  joint  and  wiped  in 
the  usual  manner.  The  compound,  at  a  temperature  of 
150  to  175  degrees  Cent.  (325  degrees  Fahr.),  is  poured 
through  one  opening  in  the  lead  sleeve  until  it  flows 
out  of  the  other  opening;  after  standing  for  a  half  hour 
or  more,  the  sleeve  is  again  refilled  and  then  sealed 
with  lead  patches.  The  principal  points  in  the  making 
of  this  joint  seems  to  be  the  exclusion  of  moisture  and 
the  use  of  a  compound  which  will  not  absorb  moisture, 
has  high  dielectric  strength,  will  not  attack  the  con- 
ductors or  their  insulations,  and  careful,  thorough 
workmanship  throughout. 

After  making  a  joint  complete,  it  is  advisable  to  let 
it  stand  for  from  two  to  six  weeks  and  then  tap  the 
lead  sheath  and  refill  it  with  compound.  Experience 
shows  that  in  joints  and  end-bells  the  compound  grad- 
ually shrinks  or  works  itself  away  into  the  insulation 
so  that  it  is  necessary  to  add  additional  compound 
from  time  to  time. 

Specifications.  Although  some  suggested  form  of 
specification  for  cable  insulation  would  be  appropriate 
and  desirable  in  a  book  of  this  character,  the  author 
has  purposely  omitted  any  such  specifications  for  the 
following  reasons  : 

First.  General  information  regarding  current  prac- 
tice and  the  latest  developments  in  the  insulating  art, 


7  2  CABLE  INSULA  TION. 

will  much  better  qualify  the  competent  engineer  to 
draw  his  own  specifications  best  adapted  to  the  especial 
requirements  of  his  particular  case  than  any  general 
specification  that  does  not  attempt  to  mention  the  de- 
tails which  are  necessary  to  make  specifications  of 
value. 

Second.  Owing  to  the  different  characteristics  of 
the  dielectrics  used,  the  wide  variance  in  conditions  of 
installation  and  voltage  operation,  any  single  specifica- 
tion would  be  too  general  to  be  of  value  and  detailed 
specifications  for  all  conditions  would  result  in  almost 
a  book  of  specifications. 

Third.  Because  of  the  comparatively  brief  experi- 
ence with  and  relatively  inexact  knowledge  of  the 
properties  of  the  various  dielectrics,  engineers  of 
standing  differ  considerably  as  to  the  requirements  and 
tests  which  a  given  set  of  specifications,  should  include. 
Until  there  is  a  greater  unanimity  of  opinion  than  thus 
far  shown — as  evinced,  for  example,  by  the  failure  of 
the  committee  appointed  by  the  American  Institute  of 
Electrical  Engineers,  to  recommend  specifications — it 
is  probably  desirable  that  each  engineer  use  his  own 
best  judgment. 

Fourth.  Specifications*  for  cables  insulated  with 
30  per  cent  rubber  compound,  have  been  agreed  upon 
by  representatives  of  the  Engineers'  Association. 
Similar  specifications  for  paper-insulated  cables  have 
also  been  issued  by  companies  engaged  in  that  line  of 


*  Copies  of  which  may  be  had  upon  application  to  almost  any  of  the 
prominent  cable  manufacturers. 


CABLE  INSULATION.  73 

manufacture.  These  specifications,  having  been 
drawn  up  by  the  representatives  of  the  manufacturers, 
stipulate  tests  which  may  fairly  be  called  conserva- 
tive ;  but  taken  in  connection  with  the  Standardization 
Rules  of  the  A.  I.  E.  E.,  adopted  June  21,  1907,  are  an 
excellent  guide  to  specification  writing. 

Practical  Commercial  Potentials  Elaborate  calcu- 
lations have  been  made  by  certain  engineers  tending  to 
show  that,  on  account  of  the  increased  cost  of  insula- 
tion with  increased  potentials,  25,000  volts  is  as  high  an 
e.  m.  f.  as  is  commercially  and  financially  economical 
to  use  in  underground  transmission.  In  order  to  check 
these  theoretical  conclusions  it  is  only  necessary  to 
compare  the  bona  fide  prices  of  cables  that  may  be  ob- 
tained for  different  voltages.  An  examination  of  the 
curves  (see  Fig.  4),  which  are  based  on  recent 
quotations,  shows  that  as  great  advantage  can  be  ob- 
tained by  increasing  the  potentials  from  20,000  or 
25,000  volts  to  30,000  or  35,000  volts,  as  is  gained  by 
increasing  the  voltage  from  10,000  or  11,000  volts  to 
20,000  or  25,000  volts.  This  is  so  because  the  cost 
increases  in  the  same  ratio  as  the  voltage,  both  below 
and  above  25,000  volts,  as  indicated  by  the  curves  being 
straight  lines.  The  practical  limits  to  continued  in- 
crease in  voltage  are  the  mechanical  difficulties,  the 
most  obvious  being: — 

(a)  Handling  cables  of  such  great  diameter  as 
would  result  from  the  use  of  even  the  best  dielectrics 
at  present  known. 


74  CABLE  IXSULATIOX. 

(b)  Necessity  for  the  use  of  larger  ducts  requiring 
special  construction. 

Inside  measurements  of  the  larger  vitrified  ducts 
generally  used  at  present,  are  about  3j/>  inches,  this 
dimension,  therefore,  determines,  as  about  3  inches 
the  maximum  diameter  of  cable  that  can  be  used, 
allowing  for  necessary  play  in  drawing  in  the  cable. 
As  a  general  proposition  there  is  no  reason  why  4-inch 
vitrified  ducts  should  not  be  installed  in  many  large 
cities,  as  the  increased  expense  would  be  but  an  inap- 
preciable percentage  of  the  total  cost  of  the  complete 
conduit  and  the  future  advantage  may  be  considerable. 
The  tendency  in  this  direction  is  indicated  by  the  re- 
cent availability  of  ducts  having  an  inside  diameter  of 
about  4  inches.  Another  difficulty  to  be  overcome  in 
exceeding  the  diameter  of  3  inches  for  a  completed 
cable,  is  found  in  the  leading  machines  at  present 
available,  which  cannot  handle  a  cable  much  larger 
than  3  inches  in  diameter. 

Assuming  three  inches  as  the  limit  of  the  outside 
diameter  of  a  complete  cable  to  be  installed  in  standard 
three  and  one-half  inch  ducts,  or  three  and  three-eighth 
inches  for  the  cable  with  four  inch  ducts,  which  are 
now  regularly  in  stock,  and  accepting  dielectrics  at 
present  used,  it  may  be  both  practically  and  com- 
mercially advisable,  even  with  an  advance  over  present 
prices  of  lead,  copper  and  insulating  materials,  to  em- 
ploy as  high  as  35,000  volts  for  underground  transmis- 
sion. With  improved  or-  "graded"  dielectrics,  pro- 
vided the  amount  of  power  being  transmitted  does  not 


CABLE  INSULATION.  75 

require  the  use  of  conductors  of  too  large  cross-section, 
three-conductor  cables  for  higher  than  35,000-volt  ser- 
vice would  seem  advisable.  Under  certain  conditions, 
as  in  the  case  of  underground  connection  between 
a  substation  in  the  centre  of  a  city  and  the  end  of  a 
high-tension  aerial  transmission  line  operating  at 
50,000  volts  or  75,000  volts,  the  use  of  such  voltages  on 
single-conductor  underground  cables  could  be  recom- 
mended. 

"On  comparatively  short  lengths  under 
ground  or  under  water,  as  a  part  of  a  long 
overhead  transmission  line,  cables  operating  at 
40,000  volts  can  be  used."* 

The  use  of  single-conductor  cables  for  the  higher 
voltages  means  a  very  appreciable  increase  in  cost 
as.  compared  with  a  three-conductor  cable  for  the 
same  voltage,  enclosed  in  a  single  lead  sheath.  A 
compensating  advantage,  however,  in  the  use  of  sep- 
arately insulated  conductors  is,  that  fewer  reserve  con- 
ductors need  be  installed.  For  example,  five  single 
conductors,  each  in  a  separate  duct,  would  probably 
afford  as  much  reserve  insurance  as  two  three-con- 
ductor cables,  because,  in  case  of  a  burnout  in  a  three- 
conductor  cable,  the  use  of  the  entire  cable  would 
ordinarily  be  discontinued;  whereas,  with  single-con- 
ductor cables,  in  separate  ducts,  one  or  two  cables 
could  burn  out  leaving  the  third  for  use  with  the  other 
two  reserve  conductors. 

*  Proceedings  A.  T.  E.  E.,  January,  1909,  page  14. 


7 6  CABLE  INSULATION. 

From  the  quotations  hereafter  given  on  the  three- 
conductor,  lower  voltage  cables,  say  25,000  volts,  and 
the  single  conductor,  higher  voltage  cables,  for  ex- 
ample, 50,000  volts,  it  can  be  shown  that  where  large 
blocks  of  power  are  to  be  transmitted,  the  higher  volt- 
age cable  installation  will  cost  less  than  that  the  lower 
voltage,  without  considering  some  slight  advantage 
to  be  gained  by  the  installation  of  fewer  ducts  and  the 
less  cost  of  drawing  in  and  connecting  the  single  con- 
ductor cables. 

The  liability  of  an  increased  rate  of  depreciation  in 
the  use  of  cables  operating  at  higher  potentials  must 
properly  be  considered.  This  increased  risk  is  due  to 
electrostatic  effects  and  the  liability  of  decomposition 
in  other  than  inactive  organic  substances  used  in  the  in- 
sulating material.  Although  these  questions  have  not 
yet  been  scientifically  investigated,  the  use  and  opera- 
tion of  cables  designed  for  25,000  volts  has  shown  no 
abnormal  depreciation.  An  800  ft.  section  of  the  25,- 
000-volt  rubber  insulated  cable  installed  at  St.  Paul 
was  recently  returned  for  re-sheathing  (necessary  by 
reason  of  the  destruction  of  the  sheath  by  electrolytic 
action  of  street  railway  currents),  which  showed  the 
Insulation  was  in  every  respect  as  good  after  seven 
years  of  continuous  operation  as  when  first  installed. 


CHAPTER  V* 
METAL  IN  CABLES 


Copper.  Copper  is  used  almost  exclusively  as  the 
transmitting  medium  for  electricity  because  of  its 
strength,  malleability,  ductility,  conductivity  and  re- 
latively low  -price.  For  aerial  circuits,  aluminum  has 
been  used  to  some  extent,  but  thus  far,  scarcely  at  all 
for  insulated  conductors.  Practically  all  of  the  copper 
used  for  electrical  purposes  has  been  refined  electro- 
lytically,  and  when  soft  and  annealed  has  a  con- 
ductivity close  to  unity,  as  compared  with  Dr. 
Matthiessen's  standard ;  hard  drawn  copper  has  some- 
what greater  resistance  than  soft  copper.  The  usual 
wire  specifications  of  98  per  cent  pure  is  appreciably 
under  what  may  be  required  of  ordinary,  commercial, 
refined  copper. 

The  elastic  limit  of  copper  ranges  from  about  7,000 
pounds  per  square  inch  with  .168  inch  soft  drawn  wire, 
for  example,  to  about  40,000  pounds  per  square  inch 
with  .1046  inch  hard  drawn  wire;  that  is,  from  22  per 
cent  of  the  ultimate  tensile  strength  in  the  first  instance 
to  60  per  cent  in  the  last  instance.  These  figures  must 
be  taken  as  approximate,  because  the  elastic  limit 
varies  with  the  amount  of  drawing  and  hardening  the 
sample  has  received.  Perhaps  what  is  more  important 
than  elastic  limit  in  a  copper  conductor,  is  the  tensile 


78  METAL  IX  CABLES. 

strength,  which  for  annealed  copper  is  usually  taken  at 
about  30,000  pounds  per  square  inch,  and  for  hard 
drawn  copper,  at  about  60,000  pounds  per  square  inch 
at  70  degrees  Fahr.  The  range  of  temperature  encoun- 
tered in  the  practical  operation  of  underground  cables 
is  too  small  to  have  any  material  effect  on  the  tensile 
strength  of  copper. 

TABLE  VI. 

COMMERCIAL  BARE  COPPER  SOLID  WIRES. 


Size 
B  &S. 


Area 
C.  M. 


6 

26,250 

5 

33,100 

4 
3 

2 
I 

41,740 
52,630 
66,370 
83,690 

O 

105,500 

oo 
ooo 

0000 

133,100 
167,800 

211,  600 

Diam. 

Resistance  at 

Inches 

bSS  F. 
Ohms  per  I  ooo  ft 

.162 

.3944 

.181 

.3128 

.204 

.2480 

.229 

.1967 

•257 

•  *56o 

.289 

.1237 

•324 

.0981  i 

•364 

.07780 

.409 

.06170 

.460 

.04893 

BREAKING  WEIGHT 


Lbs. 


1,221 
1,520 
1,890 

2,33* 
2,892 

3,565 
4,386 

5,365 
6,533 
7,9H 


Lbs.  per 
sq.  in. 

59^300 
53,500 
57,600 
55,600 
55,500 
54,2oo 
52,900 
51,300 
49,500 
47,600 


On  account  of  rigidity,  larger  copper  conductors  are 
made  up  in  the  form  of  a  stranded  cable,  consisting  of 
a  number  of  smaller  wires.  This  construction  results 
in  a  somewhat  higher  elastic  limit,  greater  tensile 
strength  and  larger  diameter,  as  compared  with  solid 
wire.  Any  number  of  wires  can  be  laid  up  to  form  a 


METAL  IN  CABLES. 


79 


cable,  but  the  size  of  the  individual  strands  and  the 
method  of  laying  them  up  so  as  to  secure  the  simplest, 
most  compact,  most  flexible  and  least  expensive  con- 
struction is  the  result  of  considerable  experiment  and 
experience.  In  the  construction  of  conductors  for  un- 

TABLE   VII. 

COMMERCIAL  BARE  COPPER  STRANDED  WIRES. 


Size  of 

Conductor 

in  C.  M. 


Number        Diam.  of 
of  Strands 

Strands        in  Inches 


Diam.  of          Resistance  at 

Cables  68°  F. 

in   Inches    Ohms  per  1,000  ft. 


4 

7 

.0771 

.231 

.2480 

3 

7 

.0866 

.260 

.1967 

2 

7 

•0975 

.292 

•  :56o 

I 

19 

.0663 

•332 

.1237 

O 

19 

.0746 

•373 

.0981 

oo 

19 

.0837 

.419 

.0778 

000 

19 

.0941 

.471 

.0617 

oooo 

19 

•1055 

.528 

.0489 

250,000 

37 

.0821 

•575 

.0414 

300,000 

37 

.0900 

.630 

.0345 

350,000 

37 

.0972 

.680 

.0296 

400,000 

37 

.1039 

.727 

.0259 

450,000 

37 

.1103 

.772 

.0230 

500,000 

37 

.  1  163 

.815 

.02071 

dergTound  cables  layers  of  copper  wires  are  placed 
around  the  core  with  a  slight  spiraling,  then  additional 
layers  are  added  alternately  spiraled  in  opposite  direc- 
tions, until  the  desired  cross-section  is  obtained.  This 
arrangement,  while  not  quite  as  flexible  or  possessing 
quite  the  tensile  strength  of  strands  made  up  into  ropes 


So 


METAL  IN  CABLES. 


and  the  several  ropes  combined  in  a  cable  permits  the 
maximum  economy  in  applying  the  insulation.     Con- 

TABLE  VIM. 

APPROXIMATE    OUTSIDE    DIAMETERS    OF 

THREE-CONDUCTOR  COPPER  CABLES. 

(}i  Lead  Throughout) 

Insulation  Thickness  on  Each  Conductor,  and  Over  Bunch 
Respectively  Equal  to 


SIZE 

5/32  +  5/326/32  4-  6/32  7/32  +  7/32  8/32  +  8/32 

10/32  +  10/32 

Diam.     Diam.     Diam.     Diam 

i         i 

Diam. 

4j 

T,735 

1,930 

2,129 

2,324  ; 

2,717 

3 

1,795 

1  ,Q9O 

2,189 

2,384 

2,777 

2 

1,864 

2>°59 

2,258 

2,453   ; 

2,845 

Ij 

i,95o 

2,145 

2,344 

2-539 

2,933 

0 

2,038 

2,233 

2,432 

2,627 

3,020 

00 

2,137 

2,332 

2,53i 

2,726 

ooo 

2,  246 

2,442 

2,640 

2,839 

oooo 

2,371 

2,567 

2,765 

2,960 

C.  M. 

I 

250,000 

2,472 

2,668 

2,866 

300,000 

2,588 

2,785 

2,983 

350,000 

2.7CO   ' 

2,895 

400,000 

2,803 

3,000 

I 

450,000 

2,898 

I 

500,000 

2,Q88 

ductors  of  large  cross-section  are  inadvisable  for  alter- 
nating currents,  unless  subdivided  into  several  ropes, 
or  even  separate  cables,  on  account  of  "skin  effect,"  in- 
duction and  increased  ohmic  losses  due  to  the  greater 
length  of  the  spiralled  strands,  which  the  current  fol- 
lows. 


METAL  IN  CABLES.  81 

Aluminum.  The  relatively  large  diameter  of 
aluminum  conductors  compared  with  those  of  copper, 
where  the  prices  for  equal  conductivity  in  these  metals 
have  been  maintained  fairly  closely — as  has  been  the 
case  in  this  country — has.  prevented  any -extended  use 
of  insulated  aluminum  conductors.  The  expiration,  at 
about  this  time,  of  the  patents  which  contain  the  funda- 
mental claims  covering1  the  production  of  aluminum 
and  the  recent  dissolution  of  the  agreement  holding  up 
prices  in  Europe,  has  resulted  in  a  marked  drop  in 
prices  of  aluminum,  both  abroad  and  in  America, 
with  every  prospect  of  a  continued  range  of  prices 
being  maintained  at  a  lower  level  than  ever  before. 
The  result  is  that  the  manufacturers  of  insulated 
conductors  have  taken  up  the  furnishing  of  alumi- 
num cables,  which  are  now  available  at  prices 
particularly  favorable,  as  against  copper  cables.  For 
example,  a  recent  quotation  on  1,000,000  c.  m.  copper 
cable  insulated  to  4/32  inch  with  1/8  inch  lead  sheath, 
was  given  as  76  cents  per  foot,  whereas  a  1,600,000  c. 
m.  aluminum  cable  (having  the  same  conductivity  as 
1,000,00(5  c.  m.  copper),  with  the  same  thickness  of  in- 
sulation and  sheath,  was  offered  at  65  cents  per  foot. 
Such  a  reduction,  of  from  12  to  13  per  cent  in  the  cost 
of  cable  for  a  given  installation,  will  doubtless  result  in 
the  wide  use  of  aluminum  insulated  cables.  In  the 
example  cited  above,  the  increased  diameter  of  the 
aluminum  cable,  as  will  sometimes  be  the  case,  was  not 
objectionable  as  only  one  cable  would  be  installed  per 
duct.  In  the  case  of  the  copper  cables,  the  diameter 


82 


METAL  IN  CABLES. 


was  1  5/8  inch,  and  in  the  case  of  aluminum.  2  inches, 
an  increase  of  3/8  inch  in  diameter,  which  is  not 
sufficient  to  make  the  drawing  in  laborious  or  in- 
jurious. The  relatively  increased  diameter  of  the 
aluminum  gives  an  increased  heat  radiating  surface 
and  thus  permits  a  larger  current  capacity  without 


increasing  the  "skin  effect." 


TABLE   IX. 

COMPARATIVE  DIAMETERS  OF  BARE  COPPER  AND 

ALUMINUM  STRANDED  WIRES  HAVING 

THE  SAME  CONDUCTIVITY. 


COPPER 

ALUMINUM 

Cir.  Mils. 

No.  of 

Strands 

O   D. 

Cable 

Cir.  Mils. 

No.  of 
Strands 

O.  D. 

Cable 

105,500 

J9 

•373  in. 

168,800 

J9 

.470  in. 

133,100 

J9 

.419    " 

212,960 

J9 

•529    " 

167,800 

i9 

.470    " 

268,480 

J9 

•595    " 

21  I,  6OO 

19 

.528    " 

338,56° 

J9 

.668   " 

250,000 

37 

•575    " 

400,000 

37 

.728    " 

300,000 

37 

.630    " 

480,000 

37 

•797    " 

350,000 

37 

.681    " 

560,000 

37 

.861    " 

400,000 

61 

.729   » 

640,000 

37 

.921    " 

45O,OOO 

61 

•773    " 

720,000 

37 

•  977    ' 

500,000 

61 

•8I5    " 

800,000 

37 

1.029    " 

To  facilitate  the  use  of  aluminum  cables,  which  can- 
not be  very  satisfactorily  soldered,  improved  methods 
of  jointing  have  been  developed.  A  particularly  suc- 
cessful form  of  joint  is  known  as  the  ''compression 


METAL  IN  CABLES.  83 

joint,"  which  is  a  sleeve  carrying  enlargements  that  are 
forced  to  flow  into  and  among  the  strands  of  the  cable 
by  means  of  a  small  hydraulic  press,  so  that  when  com- 
plete the  conductivity  of  the  joint  is  as  good  as  that  of 
the  cable  itself. 

As  there  is  one  particular  diameter  of  copper  con- 
ductor which  is  cheapest  for  each  given  voltage,  it  fol- 
lows that  if  less  power  is  being  transmitted  than  cor- 
responds to  the  proper  diameter  for  the  voltage  as- 
sumed, or  if  potential  stress  at  the  inmost  layer  of  the 
insulation  exceeds  the  dielectric  strength  of  the  ma- 
terial— so  that  the  insulation  will  break  down — it  is  evi- 
dent that  aluminum  could  be  profitably  substituted  for 
a  copper  conductor. 

The  coefficient  of  expansion  of  aluminum  and  lead 
are  nearly  alike  thus  making  them  valuable  to  associate 
together  in  cable  manufacture,  in  order  to  avoid  in- 
ternal strains  by  reason  of  change  in  temperature. 

Tin  and  Lead.  In  insulating  copper  conductors  with 
rubber,  it  is  usually  considered  necessary  to  tin  them  in 
order  to  prevent  any  free  sulphur  left  in  the  rubber 
from  attacking  the  copper.  To  this  same  end  a  thin 
layer,  1/64  to  1/32  of  an  inch  in  thickness,  of  soft,  pure 
rubber  or  rubber  compound  containing  no  sulphur,  is 
used  by  some  manufacturers,  next  the  conductor,  as  an 
additional  preventive  in  keeping  the  sulphur  away  from 
the  copper.  With  any  except  conductors  of  very  small 
diameter,  this  use  of  pure  rubber  is  probably  a  need- 
less expenditure  of  care  and  money;  because,  if  the  con- 


84  METAL  IN  CABLES. 

ductor  is  carefully  tinned  and  the  rubber  properly  vul- 
canized the  chance  for  sulphur's  attacking  the  copper  is 
very  small  on  two  accounts  :  first,  even  if  there  were 
imperfections  in  the  tin  and  the  sulphur  gets  through 
the  imperfections,  the  amount  of  copper  degraded  will 
be  so  small  relatively  that  the  conductivity  of  the  con- 
ductor— except  with  possibly  the  very  smallest  con- 
ductors— will  not  be  reduced,  as  a  practical  matter ;  and 
second,  the  amount  of  free  sulphur  in  properly  vulcan- 
ized rubber  insulation  is  so  small  that  with  conductors 
of  large  cross-section  even  not  tinned  at  all,  the  extent 
of  damage  to  same  would  probably  be  immaterial. 
"With  paper,  where  no  sulphur  is  present,  tinning  is  not 
necessary  and  is  not  resorted  to.  "With  cambric  insu- 
lation, a  "separator"  of  neutral  material  is  employed  to 
prevent  anything  in  the  varnish  attacking  the  copper. 

Some  tin  is  usually  alloyed  with  the  lead  used  for 
the  outside  sheath.  Lead  alloyed  with  tin  makes  a 
harder  sheath  and  one  less  liable  to  injury  from  contact 
with  the  sharp  projections  or  edges  encountered  in 
drawing  into  conduits.  The  amount  of  tin  specified  for 
cable  sheathing  is  usually  not  less  than  1  per  cent  or 
more  than  5  per  cent.  As  a  practical  matter,  1  per  cent 
is  a  rather  small  quantity  and  2  per  cent  as  a  minimum 
writh  3  per  cent  as  a  maximum  make  desirable  alloys; 
5  per  cent  is  apt  to  make  the  sheath  too  stiff  and  brittle. 
In  some  instances,  purchasers  require  that  the  lead 
sheath  be  dipped  in  a  tin  bath,  with  the  evident  purpose 
of  making  a  hard  exterior,  which,  while  affording  a 
finished  surface,  is  probably  too  thin  to  prove  much  of 


METAL  IN  CABLES.  85 

a  mechanical  protection,  but  which  doubtless  fully  pro- 
tects lead  against  carbonic  acid  gas  or  other  deleteri- 
ous products  which  may  attack  the  lead,  and  is  there- 
fore desirable  under  certain  conditions  of  installation. 

Chemically  pure  lead  is  both  relatively  expensive 
and  difficult  to  secure,  it  is  so  soft  and  would  so  soon 
become  friable  and  weakened  by  combination  with  car- 
bonates or  other  deleterious  substances  that  the  com- 
mercial lead,  which  usually  contains  some  antimony 
and  other  impurities,  is  fortunately  a  much  better 
material. 

The  proper  thickness  of  lead  sheath  varies  somewhat 
with  the  character  of  service  to  be  met  and  the  type  of 
insulation  employed  :  but  particularly,  with  the  size  and 
weight  of  cable  on  which  the  sheath  is  used.  For  small 
rubber  or  cambric  insulated  cables,  1/16  inch  lead  is  a 
sufficiently  heavy  sheath,  while  perhaps  something 
thicker  in  the  case  of  paper  insulation  should  be  em- 
ployed. With  large  insulated  cables,  it  may  be  neces- 
sary to  use  a  sheath  as  thick  as  3/16  inch,  but  anything 
heavier  than  this  is  apt  to  make  a  very  stiff  cable.  A 
sheath,  1/8  inch  thick  will  be  found  satisfactory  for  the 
usual  weights  of  cable,  and  normal  conditions  of  under- 
ground installation. 

As  the  lead  sheath  is  put  on  cables  for  the  purpose 
of  protecting  the  insulation,  it  is  essential  that  the  lead 
be  applied  with  uniform  thickness  and  absolute  free- 
dom from  imperfections  in  its  continuity.  If  the  tem- 
perature of  the  lead  in  the  leading  machine  is  too  high, 
the  insulation  is  not  only  likely  to  be  injured,  but  the 


86  MHTAL  IN  CABLES. 

sheathing-  will  not  be  uniform ;  and  if  the  temperature 
is  too  low,  the  sheathing  is  apt  to  contain  air  holes  or 
split  when  the  cable  is  bent.  As  the  chain  is  only  as 
strong  as  its  weakest  link,  the  absolute  integrity  of 
the  lead  sheath,  particularly  with  paper  insulated 
cables,  is  absolutely  essential. 

The  lead  sheath  is  really  a  more  delicate  part  of  a 
cable  than  is  usually  considered,  because  it  is  rather 
wreak  mechanically,  easily  destroyed  by  electrolysis, 
disintegrated  by  mechanical  action  or  relatively  small 
temperature  rises,  and  attacked  by  at  least  one  insect 
found  both  abroad  and  in  the  United  States.  It  has 
been  claimed  that  90  per  cent  of  all  failures  of  under- 
ground cables  has  resulted  from  breakdowns  of  one 
sort  or  another,  in  the  lead  sheaths. 

Two  or  more  conductors  of  the  same  circuit  should 
always,  if  possible,  be  placed  under  the  same  lead 
sheath,  because  currents  induced  in  the  lead  circulate 
through  the  points  of  contact  of  the  respective  cable 
sheaths,  causing  heating  or  arcs  liable  to  damage  the 
lead  or  cause  explosions  from  accumulated  gases.  The 
energy  losses  in  lead  sheaths  have  been  investigated  by 
Morris  of  England  and  Dr.  Monasch.  The  former 
found  that  with  a  given  waveform  and  cable  they  varied 
directly  as  the  length  and  .7  power  of  the  thickness  of 
sheath  and  as  the  square  of  current  and  frequency, 
and  for  "a  three-core  cable  carrying  50  amperes  per 
phase  with  a  frequency  of  60  periods  and  with  a  thick- 
ness of  insulation  between  each  conductor  .35  inch,  and 
thickness  of  sheath  .125  inch,  the  loss  in  the  lead  sheath 


METAL  IN  CABLES.  87 

was  17  watts  per  mile,"  or  with  the  ordinary  com- 
mercial three  phase  transmission  the  sheath  loss  is  an 
unimportant  percentage  of  the  total  energy  considered. 
On  the  other  hand  single  conductor  cables  carrying  al- 
ternating currents  may  have  large  voltages  and  result- 
ing currents  induced  in  their  sheaths.  Fisher*  reports 
having  obtained  "from  15  to  30  volts  per  1,000  ft.  with 
an  ordinary  lead-covered  cable,  and  in  the  case  of  a 
steel-wire  armored  cable  the  lead  volts  per  1,000  ft. 
were  100"  and  armoured  with  "two  wraps  of  steel  tape, 
350  volts,"  Under  such  conditions  the  advisability 
of  frequent  grounding  of  sheath  or  armor  is  evident. 

"^Proceeding's  A.  I.  E.  E.,  January  1908, 


CHAPTER  VI. 
HEATING  OF  CABLES 


Cables  versus  Wires.*  \Yhile  the  diameter  of  high- 
tension  transmission  conductors  for  aerial  work  is 
usually  determined  by  the  drop  of  potential  allowable, 
very  frequently  the  factor  controlling  the  cross-sec- 
tion of  underground  cables  is  the  permissible  tempera- 
ture rise  of  the  insulation,  particularly  when  a  cable  is 
installed  in  a  conduit  system  consisting  of  many  con- 
tiguous ducts.  The  same  causes  that  limit  the  carry- 
ing capacity  of  aerial  conductors  applies  to  under- 
ground conductors ;  but  they  are  aggravated  by  the  in- 
sulation surrounding  the  conductor. 

The  current  carrying  capacity  of  a  cable  depends  on, 

(a)  The    initial    temperature    of   the    medium    sur- 
rounding or  in  contact  with  the  cable. 

(b)  The  ability  of  the  surrounding  medium  to  dis- 
sipate heat. 

(c)  The  ability  of  the  dielectric  and  sheath  to  trans- 
mit heat. 

As  all  heat  generated  in  a  conductor  must  be  radi- 
ated through  the  surface  area,  and  as  this  varies  as  the 
diameter  while  the  cross-section  varies  as  the  square 
of  the  diameter,  it  is  seen  that  the  heat  radiating  sur- 
face does  not  increase  anything  like  as  rapidly  as  the 

*  The  author  uses  Cables  as  applying  only  to  insulated  conductors, 
usually  lead  covered,  and  wires  to  bare,  aerial  conductors,  whether  solid 
or  stranded. 


HEATING  OP  CABLES.  89 

conductivity  or  circular  milage,  the  result  is  that  the 
current  carrying  capacity  (cross-section)  is  limited  by 
the  heat  radiating  area  (surface),  and  in  consequence, 
all  conductors  of  large  size  must  carry  fewer  amperes 
per  circular  mil  than  small  conductors.  With  the 
light  insulation  required  for  600  volt  service  it  has  been 
found,  for  example,  that  the  practical  limit  of  size,  by 
reason  of  radiating  area,  is,  2,000,000  c.  m. 

Bare  conductors  can  usually  radiate  the  heat  gen- 
erated by  any  current  they  may  be  called  upon  to 
carry,  within  limits  of  commercial  drop  in  voltage. 
However,  on  account  of  its  greater  radiating  area  a 
single  conductor  cable  suspended  in  air  will  dissipate 
the  heat  generated  therein,  more  freely  and  maintain 
a  lower  temperature  than  a  bare  wire  similarly  lo- 
cated. With  cables,  however,  the  method  of  installa- 
tion prevents  the  free  dissipation  of  heat  generated,  so 
that  their  carrying  capacity  in  amperes  is  relatively 
larg'ely  reduced. 

Ignoring  the  change  in  resistivity  of  a  conductor,  the 
heat  developed  per  unit  of  length  is  constant,  whereas 
the  temperature  rise  is  logarithmic ;  so  that  in  case  of 
a  cable  carrying  a  constant  number  of  amperes  the 
temperature  first  rises  rapidly,  perhaps  75  per  cent 
of  the  ultimate  temperature  within  the  first  hour,  and 
then  somewhat  slowly,  depending  in  each  case  on  the 
thermal  time  constant  of  the  insulating  material  and 
reaching  the  final  temperature  after  three  to  five  hours. 

The  question  of  rise  of  temperature  in  underground 
cables  is  a  very  vital  one,  not  alone  because  the  insulat- 


90  HEATING  OF  CABLES. 

ing  qualities  of  the  dielectric  decrease  and  deteriorate 
very  rapidly  with  increase  in  temperature  but  also  be- 
cause the  alternate  expansion  and  contraction  of  the 
conductor,  dielectric  and  sheath,  with  varying  loads, 
tends  to  mechanically  injure  the  insulation  and  the 
sheath,  as  all  three  materials  have  different  coefficients 
of  expansion.  Instances  are  reported  of  the  cutting  of 
lead  sheaths,  resting  on  the  sharp  edge  of  tiled  ducts, 
by  alternate  lengthening  and  shortening"  of  a  cable 
due  to  heating  and  cooling. 

Rise  of  temperature  is  particularly  important  as  re- 
gards rubber  and  varnished  cambric  insulations,  the 
maximum  temperature  of  which  for  continuous  opera- 
tion should  probably  not  be  allowed  to  exceed  about 
65  degrees  Cent.  (150  degrees  Fahr.),  or  assuming  the 
temperature  of  the  earth  is  20  degrees  Cent.  (70  de- 
grees Fahr.,)  a  rise  of  45  degrees  Cent.  (80  degrees 
Fahr.)  is  permissible.  Although  rubber  will  transmit 
heat  somewhat  more  readily  than  paper,  cables  with 
paper  insulation  have  a  greater  current  carrying 
capacity  with  a  given  conductor  than  when  insulated 
with  rubber  or  cambric;  because  such  paper  insulation 
can  be  operated  at  a  higher  temperature,  say  80  degrees 
Cent.  (175  degrees  Fahr.) 

A.  C.  vs.  D.  C.  In  cables  used  for  continuous  cur- 
rents, heating  results  only  from  the  I2R  losses  in  the 
copper;  but  in  cables  used  for  alternating  currents 
there  are  additional  heat  losses  due  to 

(a)  effects  in  the  insulating  material  itself,  similar 
to  hvsteresis  in  iron. 


HEATING  OF  CABLES.  91 

(b)  losses  in  the  conductor  itself  or  lead  or  steel 
sheath  due  to  foucoult  currents, 

(c)  unequal  distribution  of  current  density  in  the 
cross-section  of  the  conductor,  the  density  increasing 
at   the   circumference  of   the  conductor  and  known   as 
"skin  effect." 

With  alternating  currents  and  high  potentials  the 
losses  in  the  insulation  may  be  appreciable ;  similarly, 
by  reason  of  heavy  currents  or  thick  sheaths  the  losses 
in  the  conductor  or  sheath  (see  page  86  ),  may  be- 
come noticeable ;  also,  with  conductors  of  very  large 
cross-section  where  the  current  density  is  far  from  uni- 
form, the  loss  due  to  this  "skin  effect"  (see  page  120), 
which  increases  with  frequency,  and  the  diameter  of 
the  wire,  may  become  serious ;  but  with  moderate 
potentials,  small  conductors  or  light-weight  sheathing, 
these  losses  are  usually  immaterial. 

As  determined  by  Steinmetz  and  experimentally  con- 
firmed by  Apt  and  Mauritius,  the  energy  loss  in  the 
dielectric  of  cables  is  proportionate  to  the  square  of 
the  e.  m.  f.  and  independent  of  the  load.  It  also  de- 
pends on  the  frequency,  wave  form  and  to  some  extent 
on  temperature.  Mauritius  found  that  the  loss  in  a 
certain  rubber-insulated  cable  (rubber  insulation  has 
considerable  higher  loss  than  paper  insulation)  with 
20,000  volts  impressed  for  a  cable  60  miles  in  length, 
amounted  to  28  kilowatts,  which,  howrever,  is  an  in- 
appreciable percentage  of  the  energy  being  transmitted 
in  any  commercial  installation. 

The  report  of  some  comparative  tests  on  the  New 


92  HEATING  OF  CABLES. 

York  Edison  Co.'s  high-tension  cables  are  interesting 
in  this  connection,  as  showing  the  greater  power  loss 
in  rubber  as  compared  with  paper  insulation. 

DIELECTRIC  LOSSES  IX  TRIPLEX  CABLES 
OPERATING  AT  6,400  VOLTS,  25  CYCLES. 

Paper  Cable  Rubber  Cable 

Length,  ft 10,935  24»756 

Copper,  cir.  mills 250,000  250,000 

Insulation 10/32  in.  10/32  in. 

Temperature  (about) 80°  F.  80°  F. 

Charging    current    in  amperes, 

working  conditions O'47  2.16 

Total  watts  lost 245.  333O- 

Watts  lost  per  ft 0.0224       o.  1345 

Carrying  Capacity.  Although  various  formulae 
have  been  proposed  to  determine  the  current 
capacity  of  cables,  they  depend  on  empirical  constants, 
so  that  while  the  published  results  of  experiments  are 
limited,  the  data  and  tables  based  thereon  are  more  sat- 
isfactory for  general  reference. 

When  two  or  more  conductors  are  included  under 
one  sheath,  or  several  conductors  installed  in  one  duct, 
or  when  a  number  of  ducts  are  laid  up  together,  the 
heat  generated  is  not  rapidly  transmitted  and  the  tem- 
perature of  the  cables  thus  installed  may  rise  to  an 
alarming  degree.  Two  conductors  under  a  single 
sheath  will  have  about  10  per  cent  less,  three  conduc- 
tors about  25  per  cent  less,  and  four  conductors  about 
35  per  cent  less  current  carrying  capacity  than  the 
same  conductor  installed  singly.  The  effect  of  con- 


HEATING  OP  CABLES.  93 

tiguous  ducts  on  the  heating-  of  cables  is  discussed 
on  page  104. 

In  connection  with  the  following  data  relating  to  safe 
carrying  capacity  of  cables,  it  must  be  borne  in  mind 
normal  conditions  are  assumed,  that  an  installation  in 
proximity  to  steam  pipes,  or  in  a  conduit  of  many  load- 
ed ducts,  will  reduce  the  values  given,  while  for  a  cable 
laid  across  the  bottom  of  a  deep  river,  the  values  are 
40  to  50  per  cent  too  small. 

Tests  under  the  direction  of  Mr.  Louis  A.  Ferguson, 
Vice  President  of  the  Commonwealth  Edison  Com- 
pany, Chicago,  111.,  demonstrated  that  concentric  cables 
have  less  carrying  capacity  than  twin  conductor  cables 
of  the  same  conductivity.  The  following  curves  are 
taken  from  his  paper,*  and  though  the  result  of 
measurements,  shown  in  Fig.  1,  are  based  on  paper  in- 
sulation only  4/32  inch  thick,  too  light  for  high  poten- 
tial service,  they  are  interesting  and  valuable.  The 
measurements  \vere  made  on  lead  sheathed  cable  in- 
stalled in  a  single  duct  of  vitrified  clay  pipe,  sur- 
rounded on  all  sides  with  approximately  six  inches  of 
sand. 

The  data  determined  by  Mr.  Ferguson  agrees  very 
satisfactorily,  when  allowance  is  made  for  a  cable  with 
different  insulation  in  a  single  duct,  with  measurements 
made  by  Mr.  H.  W.  Fisher,  Chief  Engineer  of  the 
Standard  Underground  Cable  Company,  Pittsburg,  Pa., 
who  carried  on  some  elaborate  experiments  to  deter- 
mine the  heating  of  cables,  under  his  Company's  direc- 

*" Underground  Electrical  Construction"  Proceedings  International 
Electrical  Congress  of  St.  Louis,  1904. 


94 


HEATING  OF  C 


320 


280 


240 


200  S- 


160  d  — 


120 


200 


400 


600 


800 


1000 


1200 


1400 


1600 


1800 


FIG.  i  —  Relation  between  current  and  temperature  of  single  con- 
ductor cables  insulated  with  ^  inch  paper,  sheathed  with  \  inch  lead, 
in  duct. 


HEATING  OP  CABLES. 


95 


o 

800 


0900.20 


0000.15 


0000.10 


1000  1200  1400  1600  1800  2000  2200 


FIG.  2 — Relation  between  increasing  current,  in  two  conductor, 
1,000,000  c.  m.  concentric  cable,  in  air  and  rise  in  temperature  and 
increase  in  resistance.  Inner  paper  wall,  ^  inch  ;  Outer  paper  wall, 
3%  inch  ;  Lead  sheath,  |  inch. 


96 


HEATING  OF  CABLES. 


25   50   75   100   125  150  175  200   225   250  275  300   325 


FIG.  3 — Relation  between  current  and  temperature  of  three  con- 
ductor cable,  insulated  with  ¥%  inch  paper  over  each  conductor  and  s\ 
inch  paper  belt,  |  inch  lead.  In  the  lower  left  hand  corner  is  shown 
the  relation  between  current  and  temperature  of  a  !So.  o  three  conduc- 
tor cable,  insulated  and  sheathed  the  same  as  the  No.  oooo  cable.  Both 
tests  were  in  ducts,  in  cold  weather,  other  cables  in  the  same  conduit 
were  not  heavilv  loaded. 


HEATING  OF  CABLES. 


97 


TABLE   X. 

RECOMMENDED  CURRENT  CARRYING  CAPACITIES 
FOR  CABLES  AND  WATTS  LOST  PER  FOOT. 

For  each  of  four  equally  loaded  single  conductor  cables  insulated 
with  7/32  inch  paper  and  having  9/64  inch  lead  covering,  installed  in 
adjacent  tile  ducts  in  the  usual  type  of  conduit  system  four  ducts  wide 
and  three  high,  where  the  initial  temperature  does  not  exceed  70 
degrees  Fahr.,  the  maximum  safe  temperature  for  continuous  operation 
being  taken  at  150  degrees  Fahr.  '1  he  figures  in  the  table  may  be 
taken  as  practically  correct  for  cables  insulated  with  7/32  inch  rubber 
or  varnished  cambric,  except  that  tempeiatures  will  then  be  about 
125  degrees  Fahr.  instead  of  150  degrees  Fahr. 


Size        Safe  Cur-  !  Watts  lost  ** 

Size' 

Safe  Cur- 

Watts lost** 

B.  &  S.  G 

rent  in 
Amperes 

per  it.  at 
150°  F. 

C.  M. 

rent  in 
Amperes 

per  ft.  at 
150°  F. 

14 

18 

•97 

300,000 

323 

4.22 

13 

21 

1.03 

400,000 

390 

4.  6  1 

12 

24                  1.09 

500,000 

45° 

4.91 

I  I 

29 

1-15 

600,000 

5°5 

5.16 

10 

33             1-25 

700,000 

558 

536 

9 

38             1-39 

800,000 

607 

5-56 

8 

45 

i-53 

900,000 

650 

5-7i 

7 

53             1-67 

1,000,000 

695 

5.86 

6 

64 

1.85 

1,100,000       740 

6.01 

5 

76 

2.08 

1,200.000 

780 

6.13 

4 

9i 

2.31 

1,300,000      820 

6.25 

3 

108 

2-54 

1,400,000 

857 

6-37 

2 

*2'5 

2-77 

1,500,000 

895 

6.49 

I 

146 

3.00 

1,600,000 

933 

6.61 

0 

168 

3-23 

1,700,000 

970 

6.73 

00 

195 

3.46 

1,800,000 

1010 

6.85 

ooo 

225            3.69 

1,900,000 

1045 

6-97 

oooo          260             3.92 

2,000,000 

1085 

7.09 

*Copyright,  by  Standard  Underground  Cable  Co.,  igo6.     Hand  Book  No.  XVII. 

**This  column  represents  the  amount  of  energy  which  is  transformed 
into  heat  and  which  must  be  dissipated. 


98  HEATING  OF  CABLES. 

tion.  The  formulae  and  tables  given  in  the  Handbook 
of  the  Standard  Underground  Cable  Company,  have 
been  found  to  give  excellent  satisfaction  in  practice  and 
are  here  reproduced  in  part,  through  the  courtesy  of 
that  Company. 

For  a  single  conductor  of  the  size  given  in  Table 
X,  two  or  more  conductors  of  smaller  size  may  be  sub- 
stituted, as  shown  in  Table  XI,  owing  to  the  fact  that 
for  the  same  temperature  rise,  more  current  can  be  car- 
ried by  using  divided  circuits. 

*TABLE  XI. 

EQUIVALENT  CONDUCTOR  AREAS. 


B   &  S.  G.  In  2  con-  In  4  con-  In  8  con    In  16  con-  In  32  con-  In  64  con- 
No,          ductors    ductors     ductors      ductors       ductors       ductors 


0000 

No  o 

No.  3 

No.  6 

No.  9 

No.  12 

No.  15 

ooo 

i 

4 

7 

10 

*3 

it> 

00 

2 

5 

8 

ii 

M 

i7 

0 

3 

6 

9 

12 

15 

18 

I 

4 

7 

10 

I  "3 

1  6 

2 

5 

8 

ii 

14 

17 

3 

6 

9 

12 

J5 

18 

4 

7 

10 

13 

16 





c 

8 

1  1 

14- 

I  7 

J 

6 

9 

12 

J5 

18 



7 

10 

I  7 

16 

8 

1  1 

14 

I  7 

*Copyright,  by  Standard  Underground  Cable  Co.,  1906.     Hand  Book  XVII. 


*TABLE   XII. 

RECOMMENDED  POWER  CARRYING  CAPACITY  IN 

KILOWATTS  OF  DELIVERED  ENERGY. 

THREE-CONDUCTOR,  THREE-PHASE  CABLES. 


VOLTS 

Size  in 

4,000 

6,600 

I  I,  OOO 

13,200   22,000 

26,400 

B.  &.  S  G. 

KILOWATTS 

6 

333 

549 

915 

1,098 

*  1,831 

2,196 

5 

395 

652 

1,087 

z,3°4 

2,i74 

2,608 

4 

473 

781 

!;301 

!,562 

2,603 

3,124 

3 

562 

927 

*,544 

1,854 

3,089 

3,708 

2 

650 

1,073 

1,788 

2,145 

3,575 

4,290 

I 

759 

I»253 

2,088 

2,5°6 

4,i76 

5,012 

0 

874 

1,442 

2,402 

2,884 

4,805 

5,768 

oo 

1,014 

1,674 

2,788 

3,347 

5,577 

6,694 

000 

1,172 

i,93J 

3,217 

3,862 

6,435 

7,724 

0000 

i,352 

2,231 

3,717 

4,462 

7,435 

8,924 

250,000 

i,5°3 

2,480 

4,132 

4,960 

8,264 

9,920 

SINGLE-CONDUCTOR  CABLES,  A.  C.  OR  D.  C. 

VOLTS 

Size  in 

Bo  Q  P 

3,3oo 

6,600 

11,000 

13,200 

22,000 

24,600 

KILOWATTS 

j      1 

6 

211    422    704    844   1,408   1,688 

5 

251 

502 

836  1  ,004   1,672   2,008 

4 

300 

601 

1,001    ,202    2,002  i  2,404 

3 

356 

7i3 

1,188 

,426   2,376   2,852 

2 

4i3 

825 

i,375 

,650   2,750   3,300 

I 

482 

964   1,  606     ,928     3,212     3,856 

O 

554 

1,109   1,848    2,2l8     3,696  i   4,436 

00 

644 

1,287  2,145   2,574   4,290   5,148 

ooo    743  |  1,485  2,475   2,970  ;  4,95°   5,940 

oooo    858  1,716 

2,860   3,432   5*720   6,864 

300,000 

i,  066 

2,132 

3,553 

4,264   7,106   8,528 

400,000 

1,287 

2,574 

4,290 

5,148   8,580   10,296 

500,000 

1,485 

2,970 

4,95° 

5,940 

9,900    II,  880 

600,000 

1,667 

3,333 

5,555 

6,666   11,110  13,332 

*Copyright,  1906,  by  Standard  Underground  Cable  Co.     Hand  Bcok  XVII. 


ioo  HEATING    OF    CABLES. 

As  Table  X  is  based  on  an  initial  temperature  of  70 
degrees  Fahr.,  in  the  surrounding-  medium,  the  capaci- 
ties therein  must  be  corrected  by  the  multipliers  given 
hereafter  for  initial  temperatures,  as  follows : 
Initial  Temp...  70  80  90  100  110  120  130  140  150 
Multipliers,  ..1.00  .93  .86  .78  .70  .60  .48  .34  .00 

While  the  carrying  capacities  given  in  Table  X  may 

seem  small,  it  should  be  remembered  that  they  are  for 

four  cables    in  adjacent  ducts ;  and    if    less    than    four 

cables   are   to  be   considered,   a   correction   as   follows 

should  be  applied  which  will  give  carrying  capacities 

more  nearly  in  accord  with  those  generally  recognized. 

No.  Cables,         i  2  4         6         8        10      12 

Multipliers,       1.30      1.16      i.oo     .88     .79     .71      .63 

The  cable  in  the  corner  duct  has,  of  course,  the  best 
carrying  capacity,  next  those  in  the  side  ducts  and 
then  those  in  the  internal  ducts  in  the  order  of  their 
proximity  to  the  outside. 

The  power  factor  assumed  in  Table  XII  is  1.00,  and 
the  values  must  be  corrected,  for  alternating  currents, 
by  multiplying  the  kilowatts  given  by  the  power  factor 
of  the  delivered  load.  The  figures  are  based  on  the. 
same  data  as  Table  X,  namely,  paper-insulated  lead- 
covered  cables  installed  in  adjacent  3-inch  standard  vit- 
rified ducts  arranged  four  wide  and  three  high  in  sec- 
tion with  an  initial  temperature  not  exceeding  70  de- 
grees Fahr.  and  allowing  a  maximum  final  temperature 
for  continuous  operation  of  150  degrees  Fahr.  The 
measurements  wrere  made  on  cables  having  14/64  inch 
paper  about  each  conductor  and  with  the  multiple  con- 


HEATING    OF    CABLES.  101 

ductor  cables  a  jacket  of  14/64  inch  around  the  bunch. 
Each  increase  of  2/64  inch  above  14/64  inch  in  the 
thickness  of  the  insulation  used  would  reduce  the  am- 
peres or  kilowatts  given  in  the  tables  by  about  1  per 
cent.  The  losses  figured  are  the  I2R  losses  with  R  as 
the  resistance  of  the  conductor  at  150  degrees  Fahr. 
No  insulation  or  sheathing  losses  are  included. 

The  following  information  is  given  by' the  General 
Electric  Company  concerning  the  temperature  rise  al- 
lowable in  three-conductor  high-tension  cables  carry- 
ing 60-cycle  alternating-current.  It  will  be  noted  that 
a  definite  number  of  amperes  is  given  and  the  tempera- 
ture rise  resulting  therefrom,  apparently  deduced.  It 
may  be  said  that  the  number  of  degrees  rise  in  tem- 
perature allowed  in  this  table  is  conservative,  and  the 
ultimate  heating  allowable  is  appreciably  less  than  is 
being  permitted  by  many  operators,  at  least  of  rubber 
and  paper  cables.  The  figures  in  the  table  are  based  on 
insulation  not  exceeding  7/32  inch  thick  about  each 
conductor  with  a  jacket  7/32  inch  thick  over  the 
bunch,  and  with  a  lead  sheath,  1/8  inch  thick  over  the 
whole.  In  connection  with  this  table  it  may  be  well 
to  again  call  attention  to  the  fact  that  while  paper  in- 
sulation may  not  transmit  heat  as  readily  as  rubber  or 
cambric,  it  may  be  operated  at  a  higher  temperature 
without  detriment,  so  that  the  carrying  capacity  of  a 
given  conductor  enclosed  in  paper  is  as  great  or 
in  some  cases  10  per  cent  greater,  than  when  insulated 
wTith  rubber  or  cambric. 

The  most  economical  size  of  cable  conductor  to  use 


102 


HEATIXG    OF    CABLES. 


has  been  stated  by  one  writer*  as  that  which  shall  have 
a  cross-se.ction  between  .1  and  .15  sq.  in.  (between  125,- 
000  and  190,000  c.  m.)  for  three-core  cables.  This  con- 

TABLE  XIII. 

CURRENT  CARRYING  CAPACITY    OF    INSULATED 
THREE-CONDUCTOR  CABLES  IN  DUCTS. 

(Initial  Temperature,  20°  C.) 


Rubber  and  Yar.  Cam. 

30°  C.  Rise 

Size  of  Cable  in 

Paper,  35°  C.  Rise 

Circular  Mils 

Amperes  on  each 

Conductor 

5OO,OOO 

440 

4OO,OOO 

360 

3OO,OOO 

290 

250,000 

250 

200,000 

2IO 

150,000 

*75 

125,  ooo 

140 

100,000 

I25 

8o,OOO 

no 

6o,OOO 

85 

40,OOO 

60 

6  B.  &  S.  solid 

40 

8  B.  &  S.  solid 

24 

10  B.  &  S.  solid 

16 

"•Copyright,  by  General  Electric  Co.,  1908.     Bulletin  4591. 

elusion  being  reached  on  the  ground  that  "it  is  more 
economical    in  first    cost   per  kilowatt    transmitted    to 

*  Proceedings  A.  I.  E.  E.,  vol.  XXVIII.,  Page  91. 


H BATING    OF    CABLES.  103 

transmit  a  certain  amount  of  power  by  means  of  a  cable 
of  this,  section  working  at  a  sufficiently  high  pressure 
to  enable  it  to  carry  the  required  quantity  of  power 
than  by  any  other  section  or  voltage."  The  deduction 
;s  based  on  the  fact  that  a  small  cable  'can  be  worked 
at  a  considerable  higher  current  density  than  a  large 
one,  for  the  same  temperature  rise. 

Temporary  Loads.  From  what  has  preceded  it  will 
be  recognized  that  due  consideration  of  the  character 
of  load  to  be  carried  by  the  cable  must  be  carefully  con- 
sidered by  the  designing  engineer,  if  he  is  to  reduce' in- 
stallation costs  to  the  minimum.  A  cable  capable  of 
carrying  a  steady  rated  load  current  may  be  amply 
large  to  carry  for  brief  periods, — for  example,  the  peak 
load  of  a  lighting  station — a  current  which  is  a  very 
considerable  percentage  greater  than  the  average  load. 
For  such  intermittent  load  service,  formulae  have 
been  developed  by  Air.  R.  Apt.  *  for  single  and  three 
conductor  cables  and  by  Mr.  William  A.  Delmar  ;-f  ap- 
plicable, however,  only  to  cables  smaller  than  No.  00 
B.  &  S.,  or  insulated  for  not  over  1,000  volts;  from 
which  it  is  possible  to  determine  the  overload  pos- 
sibilities of  a  cable. 

When  intermittent  load  service  is  contemplated, 
curves  of  safe  time-current  for  such  cables  should  be 
furnished  by  the  manufacturer. 

*  Elektrotechnische  Zeitschrift    April  18,  1908. 

f  "Short  Period  Carrying  Capacity  of  Cables",  Electrical  World, 
December  12,  1908. 


io4  HEATING    OF    CABLES. 

Ducts.  The  composition  of  the  duct  material  will, 
to  a  slight  degree,  affect  the  carrying  capacity  of  cables. 
Vitrified  ducts  conduct  away  the  heat  generated  in 
the  cables  somewhat  more  rapidly  than  wood  fibre  or 
paper  ducts ;  but  this  difference  is  minimized  and 
practically  may  be  ignored  where  the  thickness  of  the 
concrete  enclosing  the  ducts  is  one-half  inch  in  thick- 
ness or  over.  What  is  a  more  important  factor  is  the 
medium  surrounding  the  conduit  system.  The  best 
heat  transmitting  material  apt  to  be  encountered  be- 
ing water-soaked  ground  or  the  water  itself,  where 
cables  are  laid  on  the  bottom  of  rivers ;  the  poorest  be- 
ing dry  sand  with  rock  and  loam  as  intermediate. 

The  arrangement  of  the  ducts  relative  to  one  an- 
other, is  all  important  where  more  than  four  ducts  are 
installed.  It  will  be  seen  for  example,  that  the  centre 
one  of  nine  ducts,  laid  three  on  a  side,  can  only  dissi- 
pate the  heat  generated  therein  through  the  other 
ducts,  and  a  cable  in  such  a  duct  will  have  about  10  per 
cent  less  current  carrying  capacity  than  one  in  a  corner 
duct.  For  the  same  relative  position — outside  or  in- 
side— the  top  ducts  are  always  the  warmest,  on  this  ac- 
count a  horizontal  arrangement  is  preferable.  The 
most  desirable  arrangement  would  be  a  single  hori- 
zontal layer  of  ducts,  but  practically,  this  would  in- 
crease the  expense  disproportionately,  so  that  ordin- 
arily, ducts  are  arranged  two  or  three  wide  and  to  the 
depth  necessary. 

As  a  protection  against  accumulations  of  gas,  and 
also  with  a  view  to  increasing"  the  carrying  capacity  of 


H HATING    OF    CABLES.  105 

cables,  it  has  been  proposed  to  ventilate  ducts  by  the 
use  of  electrically  driven  ventilators.  As  a  general 
proposition,  the  expense  of  artificial  ventilation  would 
not  be  justified:  there  may  be  special  cases  where  it 
will  be  found  advantageous,  as  is  the  cas£  with  certain 
types  of  electrical  apparatus,  such  as  railway  motors, 
etc.,  but,  as  a  practical  matter,  the  difficulty  of  forcing 
air  into  and  through  ducts,  efficiently,  is  more  serious 
than  it  would  appear. 

The  necessity  of  not  leaving  high-voltage  cables  ex- 
posed in  manholes  has  become  generally  recognized. 
It  is  impracticable  to  continue  duct  construction  across 
the  manhole,  but  a  very  satisfactory  substitute  is  made 
in  the  use  of  spliced  tile  ducts  carried  on  shelves 
around  the  sides  of  the  manhole.  The  tile  is  furnished 
in  short,  curved  pieces  to  fit  the  bends  of  the  cablq, 
which  it  encloses  and  protects  against  arcs  or 
mechanical  damage  during  work  in  the  manhole.  In 
some  installations  asbestos  strips  about  3  inches  wide 
and  1/8  inch  thick  are  wrapped  around  the  cables 
and  then  impregnated  with  silicate  of  soda,  which 
hardens  and  serves  as  an  effective  protection  to  the 
cable,  the  whole  being  further  guarded  by  wrappings 
of  galvanized  iron  or  zinc  tape,  which,  in  every  case, 
should  be  properly  connected  to  the  lead  sheath  to 
avoid  difference  of  potential  and  electrolytic  action. 


CHAPTER  VIL 

ELECTRICAL  FORMULAE 
FOR  CABLES. 


Resistance.  The  ohmic  resistance  of  a  conductor  is 
the  same,  at  identical  temperatures,  whether  used  for 
bare,  aerial  or  insulated,  underground  transmission. 
As  is  well  known,  the  resistance  of  a  conductor  varies 
directly  as  its  length  and  inversely  as  its  area,  being  10 
international  ohms  per  mil  foot  of  soft  copper  at  51  de- 
grees Fahr.  The  resistance  of  the  conductor  of  an  elec- 
tric cable  is  relatively  small,  usually  but  a  fraction  of 
an  ohm  per  mile,  whereas  the  resistance  of  the  insula- 
tion, on  the  other  hand,  is  relatively  large,  being 
measured  in  millions  of  ohms  (megohms)  per  mile  of 
completed  cable.  The  ohmic  resistance  of  commercial 
conductors  is  conveniently  had  by  reference  to  wire 
tables,  or  may  be  measured  by  a  Wheatstone  Bridge 
and  galvanometer,  or  by  ascertaining  the  drop  in  volt- 
age with  continuous  current  in  accordance  with  the 
well-known  formula, 

E 
R  =  -T- 

Where  R  equals  the  total  resistance  in  ohms,  E 
equals  the  drop  in  potential,  through  the  length  of  the 
circuit,  I  equals  the  current  flowing,  in  amperes. 


ELECTRICAL    FORMULAE    FOR    CABLES.    107 

The  approximate  resistance  in  ohms  per  mile  of  a 
copper  conductor,  having-  100  per  cent  conductivity,  at 
20  degrees  Cent.  (68  degrees  Fahr.),  is  equal  to 
54,700  divided  by  the  circular  mil  cross-section  of  the 
conductor.  This  product  should  be  multiplied  by  1.62, 
in  order  to  obtain  the  resistance  of  an  aluminum  con- 
ductor of  the  same  size. 

The  insulation  resistance  of  a  cable  varies  widely,  de- 
pending on  the  thickness  and  quality  of  the  dielectric 
employed,  being  as  high  as  2,000  megohms  per  mile  for 
rubber  insulation  and  as  low  as  20  megohms  per  mile 
for  paper  insulation.  The  determination  of  dielectric 
resistance  is  made  by  the  use  of  a  galvanometer  and 
Wheatstone's  Bridge  in  the  usual  manner. 

Inductance.  An  electrical  current  flowing  through 
a  conductor  creates  a  magnetic  flux  about  the  conduc- 
tor, which  changes  with  change  in  the  strength  or  di- 
rection of  flow  of  the  current.  Any  change  in  the  flux 
produces  an  electromotive  force,  the  value  of  which,  in 
volts,  resulting-  from  a  change  in  the  current  at  the  rate 
of  one  ampere  per  second,  has  been  defined  as  the  unit 
of  inductance,  the  henry.  The  effect  of  inductance 
is  to  cause  the  current  to  lag  behind  the  electromotive 
force.  Inductance  may  be  measured  with  a  Wheat- 
stone  Bridge  similarly  to  ohmic  resistance  by  substitut- 
ing a  standard  of  inductance  for  that  of  resistance. 

The  inductance  for  one  wire  of  either  single-phase 
or  three-phase  circuits — which  depends  on  the  size  and 
shape  of  the  circuit,  the  cross-section  and  permeabil- 


IDS    ELECTRICAL    FORMULAE    FOR    CABLES. 

ity  of  the  conductor  and  surrounding  medium  —  may  be 
calculated  for  non-magnetic  single-phase  circuits,  by 
use  of  the  following  formula  : 

L  =   D  [.08047   +   -7392  Iog10  (-r-)     ] 


L    equals    inductance    of    a    wire,    one    mile    in 

length,   in   millihenrys. 
d  equals  distance  between   centres  of  wires  in 

inches. 

r  equals  radius  of  conductor  in  inches. 
D  equals  length  of  transmission  in  miles. 

Capacity.  The  dielectric  separating  two  conductors, 
maintained  at  a  difference  of  potential,  has  the  power 
of  holding  a  quantity  of  electricity,  which  property  is 
known  as  capacity.  The  capacity  of  a  cable  depends 
on  the  size  and  shape  of  the  conductors,  the  specific 
inductive  capacity  of  the  surrounding  medium,  and  the 
distance  from  other  conductors.  The  unit  of  capacity 
is  the  farad  (the  practical  unit,  microfarad,  is  one- 
millionth  of  a  farad),  and  is  that  capacity  which  will 
contain  one  coulomb  at  a  potential  of  one  volt.  The 
effect  of  capacity  on  a  circuit  is  to  cause  a  current  to 
flow  in  advance  of  the  electromotive  force.  With  bare 
aerial  conductors,  capacity  is  usually  insignificant,  but 
with  insulated  underground  cables,  capacity  and  it? 
effects  become  quite  marked  due  to  the  higher  specific 
inductive  capacity  of  the  insulating  material  and  the 
greater  proximity  of  the  conductor  to  earth. 

The  effect  of  capacity  is  to  produce  what  is  called  a 


ELECTRICAL    FORMULAE    FOR    CABLES.    109 

charging  current,  which,  in  cables  entirely  overcomes 
any  inductive  effects  caused  by  the  cables.  With  long 
cables  and  high  potentials,  the  charging  current  may 
become  so  large  as  to  overload  the  current  rating  of 
transformers  or  generators  until  an  inductive  load  is 
supplied. 

As  an  actual  example  of  charging  current,  the  follow- 
ing figures  from  the  St.  Paul,  25,000  volt  paper-in- 
sulated cable,  nearly  three  miles  long,  are  interesting. 
Measurements  were  made  by  a  hot  wire  ammeter  in 
series  with  one  of  the  three  cable  conductors,  and  were 
3.8  amperes  at  15,000  volts,  or  .63  amperes  per  mile,  2.4 
at  20,000  volts,  or  .84  amperes  per  mile,  and  3.0  am- 
peres at  25,000  volts,  or  1.06  amperes  per  mile,  the 
curve  of  relation  between  current  and  applied  potential 
being  a  tangent.  The  voltage  wras  supplied  from  a 
three-phase  generator  through  step-up  transformers 
with  no  other  load  than  the  cable  being  tested.  Meas- 
urements on  the  rubber  cable  showed  practically 
double  the  current  in  amperes  obtained  from  the  paper 
cable,  corroborating  the  testimony  of  other  observers 
as  to  the  relative  capacity  of  paper  and  rubber  insulated 
cable. 

The  electrostatic  capacity  of  a  single  conductor  cable 
as  measured  between  the  conductor  and  the  lead  sheath 
per  mile  may  be  expressed,  in  microfarads,  by  the  fol- 
lowing formula : 

.03 
C    r  Iog10 


no   ELECTRICAL    FORMULAE    FOR    CABLES. 

The  total  charging  current  for  a  single-  conductor 
cable  equals  : 

2  TT  f  C  E  D 
io6 

For  obtaining  the  total  capacity  per  mile,  per  wire 
of  a  three-conductor  lead-covered  cable,  sheath 
grounded,  operated  three-phase  in  delta,  Mr.  L.  Lich- 
tenstein*  has  made  some  calculations  on  which  the  fol- 
lowing formula  is  based. 

.0776  K 

I     3  a2        ~W-    a^)3       i 
log,    j-p-         -R.Z.    a/ 

The  charging  current  per  wire  for  a  three-conductor 
cable  equals  : 

2  TT  f  C   E   D 


C  equals  capacity  in   microfarads    per  wire  per 

mile. 
D    equals    the    length    of    the    transmission    in 

miles. 
R  equals  the   radius  to  the   inner  edge  of  the 

lead  sheath. 

r  equals  the  radius  of  the  conductor. 
a   equals   the   distance   from   the   centre   of  the 

three-phase  cable  to  the  centre  of  one  of  the 

conductors. 

*Elektrotechnische  Zeitschrift,  Feb.  n,  1904,  Page  106. 
"  "  "      18,      "          "      124. 


ELECTRICAL    FORMULAE    FOR    CABLES,    in 

E    equals    the    impressed    electromotive    force 

between  two  conductors, 
f  equals  the  number  of  cycles  per  second. 
K    equals    the    specific    inductive    capacity    of 

cable  insulating  material,  whicn  may  be  taken 

frem  table  hereafter  given. 

TABLE  XIV. 

RELATIVE  SPECIFIC  INDUCTIVE  CAPACITY  OF 

CABLE  DIELECTRICS  AT   15  DEGREES  CENT. 

(60  DEGREES  FAIIR.) 

Air    1.0 

India  Rubber,  pure 2.3 

India  Rubber,  vulcanized 3  to  4 

Rosin 2  to  3 

Manilla    paper,    unsized 1.8 

Paper  and  rosin  oil 2.4 

Jute  and  rosin  oil 2.7 

Shellac   3  to  4 

The  capacity  and  consequently  the  charging  current 
of  electric  cables  will  vary  decidedly  with  change  of 
temperature,  so  that  the  values  given  in  Table  XIV, 
must  be  modified  in  accordance  with  the  multipliers 
given  in  Table  XV,  in  order  to  obtain  a  correct  value  of 
K  to  be  used  in  the  formulae  given  in  the  preceding 
pages. 

The  coefficients  for  saturated  paper  give  results  ob- 
tained from  paper  impregnated  with  soft  compound. 
The  coefficients  given  for  rubber  insulation  are  aver- 


ii2    ELECTRICAL    FORMULAE    FOR    CABLES. 

ages  although  the  variations  may  be  larger  than  in- 
dicated, depending  on  the  constitutents  of  the  rubber 

TABLE    XV 

INSULATION  RESISTANCE  AND  ELECTROSTATIC 
CAPACITY  TEMPERATURE  COEFFICIENTS. 


c 

SATURATED 
PAPER 
INSULATION 

VARNISHED 
CLOTH 

RUBBER  INSULATION 

1* 

CO-EFFICIENTS 

CO-EFFICIKNTS 

CO-EFFICIENTS 

1*1 

H-o 

i 

Insulation 
Resistance 

S.  I. 
Capacity 

Insulation 
Resistance 

<3 

t 

i 

Insulation 
Resistance 

h 

-I 

60 

I. 

I. 

I. 

i. 

I. 

i. 

65 

I. 

55 

•95 

I. 

38 

•95 

I.  12 

to     i 

.15 

.99 

to  .98 

70 

2. 

36 

.89 

I. 

96 

.90 

1.25 

'*         I 

•  30 

.98 

-.96 

75 

3- 

5° 

.82 

2^ 

75 

•85 

I.46 

I 

.66 

.96 

11  -93 

80 

5- 

50 

•75 

3- 

94 

•79 

1.68 

"          2 

.26 

•  95 

"•90 

85 

8. 

20 

.67 

5- 

50 

•75 

1.97 

1  '          -> 
O 

.02 

•  93 

"  .88 

90 

12. 

7 

.60 

7- 

25 

.70 

2.29 

"      '4 

.IO 

•  92 

*«  .86 

95 

22. 

•53 

10.  6 

.64 

2.70 

"      5 

.60 

.90 

"•83 

100 

33- 

.46 

15- 

3 

.60 

3.10 

11      7 

.60 

.88 

<(  .80 

no 

71. 

•34 

30. 

6 

•50 

4.40 

"    Z5 

.OO 

.84 

"  .76 

120 

154. 

•25 

55- 

o 

.42 

6.40 

"    26 

.OO 

.80 

"  .71 

130 

314. 

.19 

125. 

•35 

9-43 

"    54 

.00 

.76 

"  .65 

140 

636. 

.14 

262. 

•31 

13.00 

"  108 

.00 

•72 

"  .60 

*Copyright,  by  Standard  Underground  Cable  Co.,  1906.     Hand  Book  No.  XVII. 

compound  and  whether  or  not  the  cables  are  lead 
covered.  Capacity  coefficients  were  determined  by  the 
discharge  deflection  method. 


ELECTRICAL    FORMULAE    FOR    CABLES.    113 

F.  J.  O.  Howe  states*  he  has  found  that  the  usual 
method  of  ascertaining  capacity  gives  good  commercial 
results  in  the  case  of  rubber,  gutta  percha  and  jute 
cables ;  but  in  the  case  of  paper  cables,  may  give  a 
value  four,  five  or  even  more  times  too  high  by  reason 
of  variation  in  the  relation  of  the  leakage  and  charge 
currents.  This  is  caused  by  the  use  of  a  softer  and 
more  oily  impregnating  compound  employed  for  the 
thicker  insulations  required  for  higher  voltages.  Mr. 
Howe  found  that  by  applying  high  potential  directly 
to  the  paper  cable,  the  capacity  of  which  it  is  desired  to 
measure,  having  a  hot  wire  ammeter  in  series,  the 
capacity  could  in  every  case  be  accurately  determined. 
Of  course,  the  objection  to  the  high  voltage  method  is 
the  danger  to  the  operator  and  the  necessity  of  running 
large  machinery. 

Mr.  Howe's  conclusions  do  not  agree  with  those  of 
American  investigators  who  find  that  the  measure- 
ments of  capacity  made  by  the  ballistic  galvanometer 
method  give  results  10  to  15  per  cent  higher  than  those 
obtained  through  the  use  of  an  ammeter  and  high 
potential,  which  method  must  of  necessity  include 
some  small  but  real  power  losses.  The  ratio  between 
capacity  as  determined  by  alternating  currents 
and  the  capacity  as  measured  by  the  discharge 
deflection  method,  usually  becomes  greater  with  in- 
creases in  the  per  cent  of  Para  used  in  the  rubber  com- 
pound. The  ratio  varying  from  .75  to  .95  at  60  cycles, 
60  degrees  Fahr.  The  ratio  with  good  paper  insulation 


*  London  Electrician,   March  20,  1908. 


n4    ELECTRICAL    FORMULAE    FOR    CABLES. 

is  usually  about  .90  and  with  varnished  cambric,  from 
.50  to  .75.*  The  smaller  the  ratio  the  greater  is  the 
liability  of  the  dielectric  to  heat  as  the  pressure  stress 
is  increased,  which  would  indicate  the  disadvantage  of 
using  cambric  for  the  higher  voltages.  The  reason  the 
capacity  measurements  made  by  a  galvanometer  in- 
crease relatively  is  due,  Fisher  states,  to  a  polarizing 
action  which  occurs  when  the  temperature  of  the  in- 
sulation is  raised. 

It  is  knowrn  that  the  insulation  resistance  of  rub- 
ber cables,  at  least,  improves  with  time  and  tests  in- 
dicate that  the  capacity  of  cables  when  newly  made 
and  measured  on  reels,  is  appreciably  higher — as  much 
as  20  per  cent — than  after  they  have  been  installed  in 
ducts  for  some  time. 

British  Insulated  &  Helsby  Cables,  Ltd.,  give  the 
following  information  regarding  capacity  of  three- 
phase,  three-core  cables  insulated  with  paper,  accord- 
ing to  the  British  Standard  Specification  for  a  delta 
connected  system.  The  British  Engineering  Stand- 
ards Committee  adopted  the  following  radial  thick- 
nesses for  jute  or  paper  dielectrics  of  three  conduc- 
tor underground  cables  with  neutral  not  grounded. 


6,600   VOLTS 

IE.OOO    VOLTS 

oize  ot  liable. 
Sq.  In. 

About 
Conductors 

Belt 

About 
Conductors 

Belt 

.025  -  .075 

.23  in.          .23  in. 

•35  in- 

•  35  in. 

.  100  =  .200          .24   " 

•24    ' 

.36  "       .36  " 

.250 

•25   ' 

•25    ' 

•37    ' 

•37   ' 

II.  W.  Fisher  Proceedings,  A.  I.  E.  E.,  Vol.  XXIV,  Page  405. 


ELECTRICAL    FORMULAE    FOR    CABLES.   115 

*TABLE   XVI. 

CAPACITY  OF  THREE-CONDUCTOR  CABLES. 


SIZE  OF  CABLE 

MICROFARADS  PER  MILE 

Sq. 
Ins. 

c    M               Voltage         One  Conductor 
against  others 

All  Conductors 
tied  together 

and  Ground 

against  Ground 

•05 

63,500             6,OOO 

.299 

•359 

.1.0 

127,000 

.388 

•465 

•  15 

190,500 

.440 

•  528 

.20 

254,000 

•493 

•592 

•  25 

318,000                                          .528 

•633 

.05 

63,500         n,ooo                .238 

.285 

.  IO 

127,000                               .290 

•348 

•  15 

190,500          "              .334 

.400 

.20          254,000 

.361 

•434 

•25 

318,000 

.387 

•465 

•05 

63,500 

20,000                .176 

.213 

.IO 

127,000 

"                            .212 

•254 

•15 

190,500 

t  t 

-238 

.281 

.20 

254,000 

(  i 

•255 

.306 

•25 

318,000 

*Copyright,  B.  I.  &  H.  C.,  LTD.     Hand  Book,  1907. 

The  above  figures  are  safe  for  individual  drum 
length,  for  a  continuous  cable  of  many  drum  lengths, 
the  figures  may  be  reduced  by  20  per  cent. 

A  comparison  of  the  capacities  of  two  large  in- 
stallations are  here  given,  both  because  interesting 
and  as  showing  the  difference  in  similar  dielectrics. 


n6   ELECTRICAL    FORMULAE    FOR    CABLES. 

TABLE  XVII. 

CABLE  CAPACITY  MEASUREMENTS. 


NEW  YORK 
EDISON  Co. 

Three-  Conductor 

250,000    c.  m  , 

5/32  +  5/32  -inch 

Insulation 

i, 8 -inch 

Lead  Sheath. 

Microfarads  per 
Mile. 

Paper      Rubber 


Between  one 
Conductor  and 

Ground 

Between  two 

Conductors 


.06 


.28 


.  10 


INTERBOROUGH  RAI-ID 
TRANSIT  Co. 


Three  Conductor,  ooo  B.  &  S. 

7/32  +  7/32-inch  Insulation 
i/8-inch  Lead  Sheath. 


Microfarads  per  Mile. 
Paper  A  Paper  B 

.139  .171 

•043  -053 


All  the  capacities  given  above,  in  microfarads,  were 
calculated  from  measurements  of  charging"  current, 
made  with  an  ammeter,  high  potential  being  applied 
directly  to  the  cables.  For  the  sake  of  clearness,  it  is 
assumed  that  between  each  conductor  and  lead  sheath, 
or  ground,  there  is  a  condenser  C^;  i.  e..  there  are 
three  such  condensers  with  a  three-phase  cable.  Also 
between  each  conductor  and  its  neighbor  is  a  second 
condenser  C2  i.  e.,  three  such  condensers  making  a 
total  of  six  condensers  in  a  three-core  cable.  The 
charging  current  per  wire,  per  mile  is  then, 


ELECTRICAL    FORMULAE    FOR    CABLES.    117 

2         7T       f      E      C,  2        7T       f      E      C,     V          3 

...  _  +  _ 

10°   I/      3  10° 

If  C  is  assumed  as  a  condenser  between  each  con- 
ductor and  the  neutral  point,  grounded,  of  a  three- 
phase  cable,  as  in  the  equation  on  page  110,  it  can 
be  shown  that  C  equals  Cj-f  3C2  of  the  preceding 
formula.  It  is  estimated  that  the  capacity  per  con- 
ductor, to  ground,  Clf  of  330  miles  of  Interborough  un- 
derground cables  is  53.9  M.  F.,  and  similarly,  the 
capacity  between  conductors,  C2,  is  16.7  M.  F.,  which 
with  11,000  volts,  25  cycles,  will  give  a  total  charging 
current  of  104  amperes  per  wire. 

The  condenser  effect  of  cables,  usually  in  series  with 
the  self-inductance  of  the  generating  system,  is  a  condi- 
tion tending  to  the  creation  of  electrical  oscillations. 
An  interesting  and  startling  display  of  the  effects  of  this 
phenomena,  which  resulted  not  alone  in  the  temporary 
shutdown  of  a  large  system  but  extensive  damage  to 
cables  and  apparatus,  occurred  in  1905  on  the  lines  of 
the  Manhattan  Elevated  Railway  of  New  York  City. 
The  existing  conditions  were  very  thoroughly  investi- 
gated from  both  the  practical  and  theoretical  stand- 
point by  Mr.  C.  P.  Steinmetz,  and  are  ably  discussed  in 
his  paper*  "High  Power  Surges,  in  Electric  Distribu- 
tion Systems  of  Great  Magnitude." 

For  information  regarding  the  theory  and  experience 
with  and  advantages  of  grounding  the  neutral  of  a  high 


*  Proceedings  A.  I.  E.  E.,  vol.  XXIV.,  page  297. 


n8   ELECTRICAL    FORMULAE    FOR    CABLES. 

tension  system,  we  would  refer  the  reader  to  the  papers 
"The  Grounded  Neutral"*  and  "Experience  with  a 
Grounded  Neutral  on  the  High-tension  System  of  the 
Interborough  Rapid  Transit  Company, "y  and  the  dis- 
cussions following  these  papers.  It  will  be  found  that 
both  theory  and  practice  differ  widely  in  this  connec- 
tion ;  some  of  the  largest  systems  operating  without 
grounded  neutral  and  other  systems  in  the  same  city 
using  the  grounded  neutral. 

Reactance.  As  compared  with  continuous  currents, 
every  conductor  offers  either  increased  or  decreased  op- 
position to  the  flow  of  alternating  currents,  due  to  in- 
ductance, "skin  effect,"  and  capacity.  The  opposition 
to  the  flow  of  alternating  currents  in  a  conductor,  aside 
from  that  due  to  ohmic  resistance,  is  known  as  react- 
ance, and  equals  2  -n-  f  L,  when  caused  by  inductance, 
and  i  when  caused  by  capacity, 

2     7T     f     C 

f  equals  the  cycles  per  second. 

L  equals  the  inductance  in  henrys. 

C  equals  the  capacity  in  farads. 

In  a  series  circuit  the  algebraic  sum  of  the  inductive 
and  capacity  reactances,  wrhich  oppose  one  another, 
gives  the  total  reactance  of  the  circuit. 

When  induction  and  capacity  reactance  are  con- 
nected in  parallel,  the  resultant  current  is  the  alge- 


"  F.  G.  Clark,  Proceedings  A.  I.  E.  F.,  vol.  XXVI.,  page  1597. 
•f  G.  I.  Rhodes,  Proceedings  A.  I.  E.  E.,  vol.  XXVI.,  page  1605. 


ELECTRICAL    FORMULAE    FOR    CABLES.    II9 

braic  sum  of  the  currents  taken  by  the  respective  re- 
actances, which  currents  are  in  opposition. 

Impedance.  Impedance  is  the  total  opposition  to 
the  flow  of  an  alternating  current  in  a  conductor  due 
both  to  the  ohmic  resistance  and  the  reactance  and 
equals,  for  a  series  circuit, : 


Theoretical  deductions  as  to  the  impedance  of  high- 
tension  underground  cable  are  complicated  by  reason 
of  the  many  variables  such  as  capacity  of  the  dielectric, 
distance  between  conductors,  diameter  of  conductors, 
diameter  of  completed  cable,  etc.,  so  that  tables  are 
much  more  convenient  and  fully  as  correct  as  far  as 
practical  results  are  concerned,  because  slight  varia- 
tions from  theoretical  assumptions,  which  are  liable 
to  occur  in  manufacture,  result  in  as  great  differences 
between  theoretical  deductions  and  actual  measure- 
ments as  between  tables  and  measurements.  On  the 
following  page  is,  given  a  table  showing  the  impedance 
for  three-conductor  cables  for  potentials  not  exceed- 
ing 20,000  volts. 

The  following  figures  are  based  on  the  use  of  var- 
nished-cambric insulation,  but  the  values  are  practically 
the  same  for  other  types  of  insulation  of  the  same 
thickness  as  specified  in  the  table.  The  conductivity 
is  based  on  pure  copper  at  75  degrees  Fahr.  (and  are 
approximately  correct  for  98  per  cent  conductivity  of 


120  ELECTRICAL    FORMULAE    FOR    CABLES. 

copper  at  65  degrees  Fahr.),  with  an  allowance  of  3 
per  cent  for  spiral  path  of  conductors  and  60  cycles 
per  second. 

*TABLE   XVIII. 

APPROXIMATE  OHMIC  RESISTANCE  AND  IMPEDANCE 
OF  THREE-CONDUCTOR  CABLES  AT  60  CVCLES. 


IMI'EDAXCK  OHMS  PER  MII.K 


Re- 

sist- 

S:ze      i  ance 
B    &  S     Ohms 

per 
i  Mile 


Working  Voltage 


5,OOO       7,OOO        IO,OOO        I5,CO"J        20.OOO 
Total  Thickness  of  Insulation,  Inches 

MY  1  3  1  6    v    1  i> 

X  6T  6T  x  ~5f 


2  i 

.850 

•859 

.863 

.867 

.872    .884 

I 

.674 

.696 

.700 

.706 

.712    .724 

O 

•535 

•547 

•552 

•558  , 

•565    -580 

00 

.424 

•439 

•444 

•452 

.460    .478 

ooo! 

•336 

•352 

•357 

•365 

•374    -396 

oooo 

.267 

.283 

.288 

.296 

,306 

•332 

2  50,  000  { 

.227 

•245 

.252 

.261 

.272 

•299 

300,000 

.188 

.210 

.217 

,227 

.241 

.270 

350,000 

.161 

.I87 

.194 

.204 

.217 

.250 

400,000 

141  : 

.166 

•174 

.185 

.199 

•234 

450,000 

.127; 

.148 

•  156 

.167 

.182 

.221 

500,000 

-H3| 

•137 

.144 

.156 

.172     .212 

"^Copyright,  by  General  Electric  Co..  iqc8       Bulletin  4591. 

Skin  Effect.  Skin  effect,  or  the  unequal  distribution 
of  current  in  the  cross-section  of  a  wire,  is  a  phenomena 
which  develops  in  connection  with  alternating  currents 


ELECTRICAL    FORMULAE    POR    CABLES.    121 

only.  The  effect  increases  with  frequency  and  with 
the  diameter  of  the  conductor ;  but  with  commercial 
frequencies  now  used  and  the  size  of  conductors  em- 
ployed in  high-tension  work,  "skin  effect"  is  practi- 
cally of  little  importance ;  for  copper*  conductors  of 
300,000  c.m.  and  frequencies  not  exceeding  60  cycles 
per  second,  the  ''skin  effect"  increases  the  ohmic 
resistance  less  than  1  per  cent.  Although  an  alumi- 
num conductor  for  the  same  resistance  is  considerably 
larger  than  a  given  copper  conductor,  the  aluminum 
conductor  will  have  no  greater  "skin  effect"  than  the 
copper  conductor. 

Lord  Kelvin  investigated  this  phenomena  of  "skin 
effect"  and  made  some  calculations,  upon  which  many 
subsequent  tables  have  been  based ;  although  experi- 
mental verification  of  them  by  later  investigators, 
seems  to  be  lacking. 

The  calculations  by  which  the  formula  for  determi- 
ning "skin  effect"  is  derived,  is  too  complex  to  be  in- 
cluded in  this  volume.  For  non-magnetic  conductors 
the  formula  is  as  follows : 

*R    =  R  +      I    (  -Q0001 95  f  Dx2 4_  rooooi 05   f  Dxt 

3R^  io9       ~>      45  Ra  io9" 

Re  equals  the  resistance  to  alternating  currents. 
R  equals  the  resistance  to  continuous  currents, 
f  equals  the  cycles  per  second. 
D  equals  the  length  of  the  conductor  in  miles. 
The   skin    effect   with    magnetic   conductors    of   the 
usual  size  is  so  great  as  to  prohibit  their  use  -for  ordi- 
nary commercial  alternating  currents. 

*Based  on  formula  in  Gerard's  Le9ons  sur  1'Electricite. 


CHAPTER  VIIL 
TESTING  OF  CABLES. 


Summary.  It  is  pretty  generally  agreed  that  it  is 
impossible  to  definitely  determine  the  merits  of  a  di- 
electric intended  for  high-tension  work  by  one  set  of 
tests — electrical,  mechanical  or  physical.  Any  set  of 
specifications  should  include  all  the  three  classes  of 
tests  named,  and  this  is  particularly  true  with  reference 
to  rubber  insulation. 

In  Chapter  IV.  on  "Cable  Insulation,"  under  the 
respective  paragraphs  referring  to  the  various  types 
of  dielectrics  employed,  information  has  already  been 
given  regarding  most  of  the  requirements  that  should 
be  covered  in  order  to  insure  high-grade,  high-ten- 
sion insulation.  The  information  given  related  more 
particularly  to  chemical,  physical  and  mechanical  tests, 
while  that  which  has  been  omitted  relates  to  electrical 
tests,  which  include, 

(a)  Measurements  of  insulation  resistance. 

(b)  Determination  of  the  dielectric  strength  of  the 
insulating  coating  by  means  of  a  disruptive  discharge. 

Ohmic  and  Puncture  Tests.  Quoting  from  the 
Standardization  Rules  of  the  A.  I.  E.  E., 

"The  ohmic  resistance  of  the  insulation  is  of 
secondary  importance  only,  as  compared  with 


TESTING    OP    CABLES.  I23 

the  dielectric  strength,  or  resistance  to  rupture 
by  high  voltage.  Since  the  ohmic  resistance  of 
the  insulation  can  be  very  greatly  increased  by 
baking,  but  the  dielectric  strength  is  liable  to  be 
weakened  thereby,  it  is  preferable  to  specify  a 
high  dielectric  strength  rather  than  a  high  insu- 
lation resistance.  The  high-voltage  test  for  di- 
electric strength  should  always  be  applied." 

Of  all  tests  suggested,  the  puncture  test  is  the  most 
important.  In  the  early  days  of  cable  manufacture, 
high  insulation  resistance  as  measured  in  megohms, 
was  considered  the  essential  of  good  cable  construc- 
tion, and  it  is  still  admitted  this  is  an  important  guide. 
But  the  ohmic  resistance  of  insulations,  particularly 
rubber,  varies  greatly  due  to  differences  in  their  compo- 
sition, change  of  temperature,  or  ofttimes  to  a  change 
in  the  testing  voltage,  particularly  with  poorer  quality 
of  insulation,  even  when  all  other  factors  remain  the 
same. 

Even  a  moderate  rise  in  the  temperature  of  rub- 
ber, for  example,  very  rapidly  reduces  its  resistance 
as  measured  in  megohms;  but  a  greater  rise  effects  the 
insulation  comparatively  slowly,  in  the  way  of  decreas- 
ing its  perforation  point,  unless  high  temperatures,  say 
100  to  150  degrees  Cent.,  for  this  particular  material, 
are  continued  for  some  time,  when  possible  chemical 
changes  may  take  place.  So,  while  this  considerable 
change  of  resistance  with  moderate  increase  of  tem- 
perature is  of  little  importance  in  practical  work, 


i24  TESTING    OF    CABLES. 

because  the  leakage  loss  will  be  an  inappreciable 
amount  of  the  energy  being  transmitted,  high  tempera- 
tures will  ruin  all  cable  insulations. 

It  is  not  difficult  to  obtain  high  megohm  measure- 
ments in  inferior  grades  of  insulation,  and  one  thou- 
sand million  megohms  per  cubic  inch  for  the  best 
grades  of  rubber  is  easily  obtainable.  Cables  may  be 
accepted  as  satisfactory  from  the  standpoint  of  insula- 
tion resistance  measurements,  provided  rubber  (30  per 
cent  Para)  shows  from  1,000  to  2,000  megohms,  and 
paper  or  cambric,  20  to  50  megohms  per  mile,  at  15 
degrees  Cent.  (60  degrees  Fahr.),  after  12  hours  im- 
mersion in  water,  with  one  minute's  electrification 
preferably  with  500  volts ;  of  course,  paper  and  cam- 
bric-insulated cables  must  not  be  immersed  in  water 
until  after  being  sheathed  with  lead.  The  ohmic 
measurements  should  be  made  after  the  puncture  tests. 
The  rate  of  change  of  resistance  with  temperature, 
for  the  best  rubber  compounds,  is  said  to  be  about  2.5 
per  cent  per  degree  Fahrenheit. 

While  it  is  important  that  the  resistance  of  a  cable 
installed  be  known  and  recorded  as  a  matter  of  refer- 
ence in  locating  faults,  and  while  it  is  generally 
recognized  that  an  insulation  which  will  withstand 
high  perforation  tests  will  usually  show  satisfactory 
ohmic  resistance,  it  is  acknowledged  that  insulation 
resistance  gives  little  indication  of  disruptive  strength. 
For  example,  in  multiple  conductor  cables  for  high 
voltage,  the  jute  filling  usually  more  or  less  separates 
the  insulation  from  the  sheath,  so  that  resistance  tests 


TESTING    OF    CABLES.  125 

may  show  up  exceedingly  well ;  but  the  jute,  of  course, 
will  not  withstand  high  puncture  tests. 

As  far  back  as  1899,  when  drawing  specifications 
for  the  25,000-volt  St.  Paul  cables,  the  writer  waived  all 
resistance  insulation  tests,  depending  rather  upon  the 
perforation  tests  to  determine  the  excellence  of  the 
insulation.  Modern  practice  concurs  in  these  views 
and  megohms  required  in  high-tension  specifications 
have  been  much  reduced  or  omitted,  as  insistence  on 
high  ohmic  requirements  is  likely  to  result  in  the 
production  of  a  non-flexible  and  brittle  insulation. 

A  cable  in  practical  use  may  be  stressed  to  once  and 
a  half  or  twice  normal  voltage  by  failure  to  syn- 
chronize generators  or  the  running  away  of  a  gov- 
ernor; but  is  only  likely  to  receive  for  brief  periods, 
excessive  voltages  which  may  be  caused  by  surges  or 
something  of  that  sort.  Fortunately,  the  ability  of  a 
cable  dielectric  to  withstand  puncture  is  a  factor  not 
alone  of  the  stress,  applied  but  also  of  the  duration  of 
that  stress;  consequently,  from  two  to  two  and  a  half 
times  normal  voltage  as  a  time-test,  with  five  to  eight 
times  normal  voltage  with  momentary-test,  should 
seem  to  meet  the  requirements  of  practical  working. 
For  high-voltage  work,  the  present  consensus  of  engi- 
neering opinion  demands  a  5-minute  puncture  test  at 
two  and  a  half  times  normal  working  voltage,  at  the 
factory,  and  twice  normal  voltage  after  installation, 
without  regard  to  the  size  of  the  conductor;  but  higher 
momentary  tests  have  not  yet  been  agreed  upon.  Re- 
duced potential  tests  for  30  minutes  or  more,  are  not 


126  TESTING    OF    CABLES. 

less  valuable  than  the  5-minute  test ;  a  time-test  of 
days  or  weeks  would  give  a  still  better  indication  of 
the  durability  of  the  dielectric. 

Too  severe  high  potential  tests  may  strain  or  weaken 
some  of  the  less  strong  particles  of  the  insulating 
material,  which  may  later  break  down  under  the 
normal  working  pressure;  hence,  moderate  increase  of 
potential  should  only  be  applied  for  time-tests  with 
higher  momentary  tests  upon  cables  completely  in- 
stalled. Further,  tests  for  perforation  should  be  ap- 
plied on  pieces  10  or  15  feet  in  length,  cut  off  for  the 
purpose  of  testing.  Cables  will  momentarily  with- 
stand much  higher  potentials  than  those  which  may  be 
applied  constantly.  For  example,  a  cable  that  will 
withstand  double  normal  voltage  may  conservatively 
be  required  to  withstand  instantaneous  applications  of 
five  times  the  normal  potential.  In  making  potential 
tests,  ample  generator  or  transformer  capacity  must  be 
provided,  otherwise  the  charging  current  may  distort 
the  electro  motive  force  wave  and  reduce  the  applied 
voltage.  The  American  Institute  of  Electrical  Engi- 
neers recommends  the  use  of  apparatus  having  four 
times  the  kilowatt  capacity  of  the  apparent  energy  re- 
quired in  making  test. 

All  tests  are  based  on  the  use  of  a  sine  wave  of 
electro-motive-force.  The  wave  form  of  the  generator 
is  particularly  important,  if  the  circuits  supplied  are 
constituted  of  cables,  as  sharp  peak  or  jagged  waves 
tend  to  produce  oscillations  or  resonance;  consequent- 
ly the  designing  engineer  should  see  to  it  that  the 


TESTING    OP    CABLES.  127 

generating  apparatus,  whether  to  be  used  for  testing 
or  operating,  is  properly  designed  to  insure  a  sine  wave 
of  electric-motive-force. 

Maintenance  of  periodic  inspections  and  moderate 
tests  are  much  more  vital  to  the  continued  operation 
of  cables  than  mere  test  at  abnormally  high  voltage. 
An  instance  could  be  cited  where  cables  tested  satis- 
factorily up  to  90,000  volts  before  breaking  down,  but 
which  later  gave  trouble  under  regular  11,000-volt 
operation.  It  is  possible  to  so  treat  some  insulations 
as  to  insure  their  withstanding  high  puncture  tests  for 
a  short  time,  although  the  same  cable  will  not  oper- 
ate continuously  and  successfully  at  lower  voltages.  In 
making  high-voltage  puncture  tests,  it  is  desirable  to 
avoid  the  use  of  a  spark-gap,  as  a  measure  of  the  po- 
tential being  applied,  because  the  breakdown  of  the  air 
gap  causes  surges  which  may  result  in  the  piling  up 
of  potential  above  what  is  desired  and  consequent 
weakening  or  damage  to  the  insulation. 

It  is  not  definitely  knowrn  just  how  important  the 
element  of  time  is  in  connection  with  breakdown  tests 
of  cables.  Just  what  relation  there  is  for  the  different 
dielectrics  between  potentials  applied  and  the  length 
of  time  of  this  application,  is  not  known. 

Most  tables  of  puncture  tests  proposed  that  have 
thus  far  appeared  in  print,  are  very  conservative. 
The  tables,  prepared  for  example,  by  Fisher,*  Lan- 
gan,f  Clark,$  the  Engineering  Standards  Committee  of 


*Proceedings  A.  I.  E.  E.,  Vol.  XXIV.,  Page  414. 

f         "  "  "    XXV.          "     200. 

"  "  "    XXV.          "     212. 


i28  TESTING    OF    CABLES. 

England — except  on  the  basis  of  their  higher  puncture 
tests  which  are  about  three  times  working  potentials — 
and  probably  also  the  Engineers'  Association  of  Wire 
Manufacturers,  give  test  voltages  too  conservative  for 
commercial  conditions ;  their  use  would  result  in  the 
requiring  of  insulations  unnecessarily  thick  and  expen- 
sive for  minimum  investment  with  reasonable  factor  of 
safety,  particularly  for  the  high  voltages.  Examina- 
tion of  the  table  on  page  28  will  show  that  at  the 
higher  voltages,  at  least,  operating  plants  are  not  re- 
quiring insulations  as  heavy  as  called  for  by  the  tables 
above  referred  to.  Some  cables  used  for  the  lower 
voltages  included  in  this  table  of  installations,  have 
intentionally  been  provided  with  insulations  suffic- 
iently heavy  to  permit  doubling  the  working  voltage, 
thus  unfairly  indicating  a  larger  factor  of  safety  than 
will  be  actually  the  case. 


CHAPTER  IX. 
COSTS. 


Total  Costs.  The  complete  cost  of  a  system  for 
underground  distribution  of  electric  energy  is  made  up 
of  two  entirely  independent  components — one  the  cost 
of  the  excavation  and  subsurface  construction,  and  the 
other  the  cost  of  the  conductors  properly  insulated, 
mechanically  protected  and  installed.  The  type  of 
subsurface  structure  varies  from  simply  a  trench  in 
which  the  electrical  cables  are  buried  to  fireproof  con- 
duits, embedded  in  cement,  which  connect  manholes 
spaced  perhaps  400  feet  or  500  feet  apart,  the  manholes 
being,  in  some  cases,  as  large  as  a  small  room  and 
costing  several  hundreds  of  dollars  each.  The  proper 
type  of  underground  structure  varies  for  different  in- 
stallations, depending  on  the  investment  allowable, 
the  protection  required  and  the  desirability  of  being 
able  to  withdraw  cables  without  disturbing  the  sur- 
face of  the  ground.  In  American  cities  conditions 
more  commonly  require  the  construction  of  con- 
duit systems,  which  are  usually  of  bituminized  wood- 
fibre  or  the  more  costly  vitrified  clay  ducts,  laid  in 
Portland  cement.  Owing  to  the  difference  of  opinion 
among  engineers  as  to  the  proper  depth  below  the 
surface  conduit  should  be  laid,  the  mixture  of  cement 
to  be  used,  the  thickness  of  the  concrete  walls  enclos- 


i3o  COSTS. 

ing  the  conduits,  the  difference  in  type  and  size  of 
manholes,  and  the  varied  costs  of  excavation  due  to 
difficulties  in  local  conditions,  such  as  traffic,  sewer, 
water  and  gas  pipes,  rock  or  water-soaked  material 
the  cost  of  labor,  and  particularly  the  widely  varying 
expense  of  similar  work  owing  to  the  varying  ability  of 
those  in  charge,  it  is  impracticable  to  obtain  average 
figures  for  conduit  construction  cost.  The  ducts  vary 
from  about  9  cents  per  foot  of  length,  under  most 
favorable  circumstances,  up  to  $2.00  or  $3.00  per  foot, 
under  most  exacting  conditions.  Similarly,  manholes 
may  cost  from  a  few  dollars  up  to  five  or  six  hundred 
dollars  each,  depending  on  size,  type  and  conditions  of 
installation.  The  only  safe  method  of  estimating  the 
cost  of  conduit  construction  for  any  given  locality,  is 
by  comparison,  item  by  item,  with  due  allowance  for 
differences,  with  costs  in  another  given  locality.*  On 
the  other  hand,  the  price  of  underground  cables  will 
be  approximately  the  same  at  a  given  time,  disregard- 
ing the  easily  ascertainable  freight  rates,  in  any  part 
of  the  United  States. 

Cable  Costs.     The  cost  of  high-tension  cables  will 
vary  somewhat  from  time  to  time,  depending  on  the 


*  For  detail  figures  on  different  methods  and  varying  costs  of  conduit 
construction  for  electric  cables,  see  Foster's  Electrical  Engineer's 
Pocketbook,  fifth  edition,  page  301;  the  Electrical  Age,  November,  1908, 
Page  260;  Proceedings  National  Electric  Light  Association,  1904, 
Appendix  A,  following  page  577;  Proceedings  International  Electric 
Congress  of  St.  Louis,  1904,  vol.  II.,  page  671. 


price  of  materials  and  labor;  but  this  variation  will  be 
considerably  less  than  might  be  expected  owing  to 
the  fact  that  the  cost  of  the  cable  of  a  particular  type 
is  made  up  of  different  items,  variations  in  the  cost  of 
which,  more  or  less,  offset  one  another:  For  example, 
the  present  base  price  of  copper  is,  say  14^  cents,  the 
highest  price  having  been  27  cents,  and  the  market 
price  of  rubber  is  about  $1.10  a  pound,  the  highest 
price  having  been  only  $1.30  a  pound;  similarly,  the 
price  of  lead  at  present  is  about  4^  cents  per  pound, 
the  highest  price  having  been  6  cents  per  pound ;  paper 
costs  8  cents  per  pound,  having  cost  as  high  as  10 
cents  per  pound ;  the  cost  of  labor  is  about  as  high 
now  as  at  any  time,  although  the  efficiency  is  some- 
what better.  From  these  figures  it  will  be  seen  that 
for  a  foot  of  three-conductor  0000  25,000-volt  rubber- 
insulated  cable,  which  at  present  prices  contains  about 
28  cents  of  copper,  23  cents  of  lead  and  140  cents  of 
rubber,  out  of  an  assumed  total  cost  of  270  cents  per 
foot;  whereas,  if  the  maximum  prices  given  above  are 
all  used,  the  total  cost  is  326  cents  per  foot,  or  an  in- 
crease of  only  56  cents,  or  about  20  per  cent,  as  be- 
tween present  prices  and  all  of  the  maximum  prices 
which  have  been  reached. 

For  the  purpose  of  investigating  the  varying  costs 
of  high-potential  cables  insulated  with  paper,  cambric 
and  rubber,  and  so-called  "graded"  insulation,  prices 
were  obtained  from  several  of  the  largest  and  most 
reliable  manufacturers  of  high-tension  cables.  As  the 
quotations  were  made  about  the  first  of  October,  1908, 


1 32  COSTS. 

it  may  be  assumed  that  the  prices  named  were  based 
on  the  cost  of  raw  material  being  about  as  given  in  the 
first  part  of  this  section.  The  prices  asked  were  on 
cables  designed  for  normal  working  potentials  of 
11,000,  25,000,  35,000  and  50,000  volts,  with  conductors 
of  No.  4  and  No.  0000  B.  &  S.  gauge.  Less  recently  a 
price  was  secured  on  a  50  sq.  mm.,  75,000-volt  cable 
with  "graded"  insulation.  The  11,000,  25,000  and 
35,000-volt  cables  were  to  be  built  with  three  con- 
ductors, each  separately  insulated  and  then  laid  up  and 
enclosed  in  a  jacket  of  the  same  insulating  material, 
the  whole  being  covered  with  1/8-in.  lead  containing  3 
per  cent  tin.  The  50,000  and  75,000-volt  cables  were  to 
be  built  with  a  single  conductor  properly  insulated  and 
covered  with  a  similar  lead  sheath.  All  of  the  prices 
quoted  were  f.  o.  b.  factory,  and  the  curves  given 
hereafter  were  drawn  by  plotting  the  various  prices 
quoted  for  the  several  characters  of  insulation,  at 
different  voltages,  and  drawing  lines,  to  represent  the 
average,  through  the  points  as  plotted,  no  allowance 
being  made  for  transportation — a  value  easily  ascer- 
tainable  for  any  given  locality — or  for  the  cost  of  con- 
necting and  drawing  the  cables  into  ducts,  which  may 
be  taken  at  from  8  cents  to  10  cents  per  foot  per  cable. 
I  assume  it  is  fair  to  conclude  that  curves  obtained 
in  this  way  correctly  indicate  the  cost  of  cables  at  in- 
termediate voltage,  which  may  be  read  from  the 
curves. 


COSTS. 


133 


SPECIFICATIONS  FOR  THREE-CONDUCTOR  CABLES 
WITH  AVERAGE  OF  PRICES  PER  FOOT. 

Each  conductor  separately  insulated  and  laid  up  with 
jute  fillers  to  make  round,  the  whole  covered  with  a 
jacket  of  insulating  material,  outside  of  which  there  is 
to  be  1/8-in.  lead  sheath,  lead  to  contain  3  per  cent 
tin.  No.  0000  conductors  to  be  stranded;  No.  4  con- 
ductors to  be  solid.  All  cables  to  be  tested  for  five 
minutes  on  twice  normal  working  voltage,  after  in- 
stallations. 


II,OOO-VOLT     25  OOO-VOLT    35  OOO-VOLT 


No.  4     No.  4-0 

No.  4 

No.  4-0 

No.  4 

No.  4-0 

iper                  $0.49     $T-03 

$0.87 

i  $i.  47 

$1.15 

$1.76 

imbric               0.77       1.49 

1.36 

2.30 

i.  80 

2.87 

abber                 1-30       1.78 

i.  80 

2.66 

2.16 

3-30 

SPECIFICATIONS  FOR  THREE  SINGLE-CONDUCTOR 
CABLES  WITH  AVERAGE  OF  PRICES  PER  FOOT. 

Each  single-conductor  cable  insulated  and  sheathed 
with  1/8-in.  lead,  lead  to  contain  3  per  cent  tin.  No. 
0000  conductors  to  be  stranded ;  No.  4  conductors  to  be 
solid.  All  cables  to  be  tested  for  five  minutes  at  twice 
normal  working  voltage,  after  installation. 


i34  COSTS. 


50,000  VOLT  75,000  VOLT 

Three  No.  4         Three  No    4-0    Three  50  sq  mm. 

$6.78 


Paper 

$3.18 

$4.12 

Graded 

3-75 

4-95 

Rubber 

6.00 

6.90 

The  prices  submitted  by  the  different  manufacturers 
on  the  lower  voltage  cables  did  not  differ  much  among 
themselves,  but  as  the  voltage  increased  the  differences 
were  more  marked.  There  was  only  one  quotation  on 
75,000-volt  cables;  all  but  one  of  the  manufacturers 
requested  to  do  so  bid  on  50,000-volt  cable ;  all  re- 
quests for  prices  at  the  lower  voltages  were  complied 
with,  and  it  is  fair  to  assume  that  all  reliable,  experi- 
enced cable  manufacturers  stand  ready  to  furnish  and 
guarantee  three-conductor  cables,  as  large  as  No.  0000 
B.  &  S.  for  tensions  as  high  as  35,000  volts. 


COSTS. 


COSTS  OF  HIGH-TENSION   UNDERGROUND 
ELECTRIC  CABLES. 


FIG.  4 


501801 


UNIVERSITY  OF  CALIFORNIA  LIBRARY 


